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AGRICULTURAL  AND  BIOLOGICAL   PUBLICATIONS 

CHARLES  V.  PIPER,  Consulting  Editob 


AN  INTRODUCTION  TO  CYTOLOGY 


°Ms  Qraw-J-I ill  Book  Qx  7m 

PUBLISHERS     OF     BOOKS      FOIO 

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1HHI 


AN  INTRODUCTION  TO 

CYTOLOGY 


BY  ^ 


LESTER  W.  SHARP  ^°l^c 


CORNELL   UNIVERSITY 


//7 


"The  most  important  discoveries  of  the  laws,  rneth<><l~ 
and  progress  of  nature  have  nearly  always  sprung  from 
the  examination  of  the  smallest  objects  which  she 
contains,  and  from  apparently  the  most  insigmficanl 
enquiries." — Lamarck,  Philoaophie  Zoologiqut . 


First  Edition 


McGRAW-HILL  BOOK  COMPANY,  Inc. 

NEW   YORK:    370  SEVENTH   AVENUE 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1921 


Copyright,  1921,  by  the 
MgGraw-Hill  Book  Company,  Inc. 


THE  MAPLE  PRESS  YORK  PA 


ytl?  "Tatter 


PREFACE 

This  book  has  been  prepared  for  students  of  the  biological  science 
who  desire  a  means  of  becoming  more  readily  acquainted  with  the  Litera- 
ture and  problems  of  cytology.     It  does  not  pretend  to  be  an  exhaustive 

treatise  for  the  use  of  experienced  cytologists,  though  it  is  hoped  thai  to 
them  also  some  of  its  features  may  be  of  service. 

For  a  number  of  years  students  of  biology,  especially  those  working 
along  botanical  lines,  have  been  faced  with  the  task  of  searching  I  orough 
a  widely  scattered  literature  for  information  on  various  cytological 
subjects.  It  is  the  purpose  of  this  book  not  to  render  the  consultation 
of  that  literature  unnecessary,  but  only  to  make  it  easier;  the  student 
can  scarcely  be  too  strongly  urged  to  derive  his  information  from  original 
sources  wherever  possible.  The  author  does  not  presume  to  replace,  bin 
rather  aims  to  supplement,  Professor  Wilson's  well  known  book,  Th> 
Cell  in  Development  and  Inheritance,  which,  though  written  twenty  years 
ago  and  with  the  emphasis  primarily  on  the  zoological  side,  will  remain 
invaluable  to  all  workers  for  many  years  to  come.  The  more  recent 
works  of  Gurwitsch  (Morphologie  una1  Biologie  der  Zelle),  Heidenhain 
(Plasma  und  Zelle),  and  Buchner  (Prakticum  der  Zellenlehre)  are  of  im- 
portance especially  to  the  zoologist. 

The  living  cell,  or  protoplast,  which  represents  the  organized  proto- 
plasmic unit  of  structure  and  function,  obviously  cannot  receive  complete 
description  in  structural  terms.  Until  a  comparatively  recent  period 
cytological  researches  dealt  primarily  with  cell  structure,  including 
particulary  the  conspicuous  changes  undergone  by  this  structure  in 
connection  with  the  reproduction  of  the  cell  (cell-division)  and  <>!  the 
multicellular  organism  (maturation  and  fertilization).  A  gradual  shifting 
of  emphasis  has  since  led  to  the  opening  of  fruitful  fields  in  other  direc- 
tions, and  the  important  results  already  achieved  have  shown  wit  h  increas- 
ing clearness  the  need  for  a  closer  acquaintance  with  the  physiological 
aspects  of  cell  activity,  not  only  in  metabolism  and  growth,  bu1  also  in 
the  reproductive  phases  of  the  life  cycle.  The  present  work,  though 
dealing  mainly  with  the  structural  aspects  of  the  subject,  may  aid  indi- 
rectly in  fulfilling  the  above  need  by  making  the  prerequisite  data  ol  cell 
morphology  more  readily  available. 

Throughout  the  book,  which  in  many  of  iis  chapters  treats  chiefly 
of  the  plant  cell,  attention  is  focussed  upon  the  protoplasi  :  the  fdl  wall 
is  given  only  brief  consideration,  since  it  plays  :i  relatively  minor  rdle  in 
the  processes  of  particular  interest  to  the  cytologisl  at  the  presenl  time. 

vii 


yjjj  PREFACE 

Because  of  their  fundamental  importance  in  connection  with  the  problems 
confronting  the  geneticist,  the  phenomena  of  nuclear  division,  chromo- 
some reduction,  and  fertilization  arc  described  with  considerable  fullness, 
and  their  relation  to  the  problems  of  heredity  is  taken  up  in  five  special 
chapters.  With  regard  to  many  of  the  subjects  treated,  it  has  not  been 
found  possible  to  formulate  final  conclusions,  since  in  many  cases  nothing 
more  than  tentative  general  statements  are  warranted  by  the  facts  in 
our  possession.  In  some  chapters  little  more  than  catalogs  of  conflicting 
opinions  can  be  given,  but  in  such  a  form  the  state  of  certain  questions 
is  not  inaccurately  represented.  The  student  entering  upon  the  field  of 
cytology  will  be  impressed  by  the  large  number  of  special  points  which 
remain  undetermined  and  general  questions  which  await  adequate 
answers.  If  he  can  look  upon  cytology  as  a  developing  science,  and  if 
he  has  reached  the  stage  at  which  he  no  longer  demands  categorical 
answers  to  all  his  questions,  this  book  will  be  of  interest  to  him  as  much 
for  the  problems  it  raises  as  for  those  it  helps  to  solve.  Not  the  least  of 
its  functions  is  to  indicate  lines  of  research  along  which  he  can  hope  to 
make  contributions  to  the  subject. 

In  compiling  his  materials  the  author  has  not  hesitated  to  draw  very 
freely  upon  the  writings  of  others.  In  many  cases  where  direct  quotation 
is  not  made,  the  language  of  the  originals  has  been  closely  followed  in 
order  to  lessen  the  liklihood  of  misrepresentation.  His  great  debt  to 
Professor  Wilson's  book  will  be  apparent  to  all  those  familiar  with  that 
admirable  work.  The  majority  of  the  diagrams  and  a  number  of  the 
other  figures  are  new.  Most  of  the  latter,  however,  have  been  redrawn 
from  works  cited  in  the  text,  not  only  that  the  value  of  the  book  may  be 
enhanced  by  the  presence  of  authoritative  illustrations,  but  also  that  the 
student  may  be  encouraged  to  become  more  familiar  with  the  original 
papers.  The  general  systematic  positions  of  organisms  indicated  in  the 
text  by  their  scientific  names  only  may  be  ascertained  by  referring  to 
the  generic  names  in  the  index. 

The  illustrations  are  largely  the  work  of  Miss  Mildred  Stratton,  in 
whose  skill  and  spirit  of  cooperation  the  author  has  had  invaluable 
assistance.  The  criticisms  of  the  text  kindly  given  by  Professor  C.  J. 
Chamberlain  of  the  University  of  Chicago  and  Professor  R.  A.  Emerson  of 
Cornell  University  have  been  very  highly  appreciated.  Acknowledge- 
ments are  also  made  to  the  author's  other  colleagues  for  their  advice  and 
continued  encouragement.  Further  criticisms  looking  toward  the  im- 
provement of  future  editions  will  be  welcomed. 

L.  W.  S. 
Ithaca,  New  York, 

September  8,  1920. 


PREFACE  ix 


Note 


Less  than  two  months  after  the  completion  of  the  texl  of  tins  book 
the  author  has  received  copies  of  the  two  now  English  works  on  cytology: 
W.  E.  Agar's  Cytology,  With  Special  Reference  to  the  M<  tazoan  NucU  us  and 
L.  Doncaster's  An  Introduction  to  the  Study  of  Cytology.  Both  deal 
almost  exclusively  with  animal  cytology,  the  firsl  being  valuable  for 
its  account  of  chromosome  behavior  in  animals,  and  the  second  for  its 
discussions  of  gametogenesis,  fertilization,  parthenogenesis,  and  Bex- 
determination.  These  works,  together  with  the  botanical  portions  of 
the  present  volume,  should  make  an  acquaintance  with  the  general  field 
of  cytology  much  more  readily  attainable. 


CONTENTS 

Preface vn 

CHAPTER  I 

Historical  Sketch 1 

The  discovery  of  the  cell — Preformation  and  epigenesis     Kaily  theories 
cell-formation — Early  observations  on  the  cell  contents — The  foundation 
of  the  cell  theory — Elaboration  of  the  cell  theory — The  protoplasm  doctrine 
— The  new  conception  of  the  cell — Fertilization  and  embryogeny     The  be- 
ginning of  the  modern  period  in  cytology — Bibliography  1. 

CHAPTER   II 

Preliminary  Description  of  the  Cell 

Description  of  the  cell — The  differentiation  of  cells — Bibliography  2. 

CHAPTER  III 

Protoplasm  

Physical  properties — Protoplasm  as  a  colloidal  system — Microdissection 
Chemical   nature    of   protoplasm — Varieties    of    protoplasm — The    plasms 
membrane — Protoplasmic  connections — Vacuoles — Protoplasm  aa  the  sub- 
stratum   of    life — Micromeric   theories — Chemical    theories — Conclusion 
Bibliography  3. 

CHAPTER  IV 

The  Nucleus 59 

Occurrence — General  characters — Nucleoplasmic  ratio — Structure  of  nucleus 
— Nuclei  of  bacteria  and  other  protista — The  function   of  the  nucleus 
Bibliography  4. 

CHAPTER  V 

The  Centrosome  and  the  Blepharoplast 

The    centrosome — Occurrence    and    general    characters     Individuality 
Centrosomes    in   Algae— Fungi — Bryophytes— Conclusion     The    blepharo- 
plast— Occurrence — In    flagellates— In    fchallophytes     En     byrophytes     In 
pteridophytes — In  gymnosperms — In  animals— Conclusion     Bibliograph 

CHAPTER  VI 

Plastids  and  Chondriosomes 

Plastids — General  nature  and  occurrence— Leucoplasts     ( Jhromatophores 
Starch— The    pyrenoid— Elaioplasts    and    oil    bodies     The    eyespol     The 
individuality  of  the  plastid— Chondriosomes     General  nature  and  occur- 
rence— Physico-chemical    nature— Origin    and    multiplication     Function 
Relation  of  chondriosomes  to  plastids— Conclusion     Bibliography  6. 

CHAPTKK    VII 

1  ■ ;  •  ; 

Metaplasm;  Polarity 

Metaplasm— Extruded  chromatin-  The  senescence  of  the  cell     Polarity 

Metabolic  gradient     Bibliography  7. 

\i 


xii  CONTENTS 

CHAPTER   VIII 

Page 

Somatic  Mitosis  and  Chromosome  Individuality 143 

Somatic  mitosis — Preliminary  sketch  of  mitosis — Detailed  description  of 
the  behavior  of  the  chromosomes  in  somatic  mitosis — Chromomeres — 
Summary — The  individuality  of  the  chromosome — The  frequent  persistence 
of  visible  chromosome  Limits  in  the  resting  reticulum — Prochromosomes — 
Persistence  of  parental  chromosome  groups  after  fertilization — Size  and 
shape  of  chromosomes — Chromosome  number — Discussion  and  Conclusions 
— Bibliography  8. 

CHAPTER  IX 

The  Achromatic  Figure,  Cytokinesis,  and  the  Cell  Wall 175 

The  achromatic  figure — In  higher  plants — In  animals — Intranuclear 
figures — Origin  of  the  figure — The  mechanism  of  mitosis — Cytokinesis — 
In  thallophytes — In  microsporocytes — In  animals — Mechanism  of  furrowing 
-The  cell  wall — The  primary  wall  layer — Secondary  and  tertiary  wall 
layers — The  physical  nature  of  the  cell  wall — The  chemical  nature  of  the 
cell  wall — The  walls  of  spores — Bibliography  9. 

CHAPTER  X 

(  )ther  Modes  of  Nuclear  Division 202 

Cyanophycea? — Protozoa — Other  cases  in  plants — Amitosis — Amitosis  and 
heredity — Bibliography  10. 

CHAPTER  XI 

The  Reduction  of  the  Chromosomes 219 

Discovery — The  stage  in  the  life  cycle  at  which  reduction  occurs — The 
meaning  of  reduction — Interpretations  based  on  Weismann's  theory — 
Somatic  and  heterotypic  mitoses  compared — Modes  of  chromosome  reduc- 
tion— Scheme  A — Scheme  B — Comparison  of  Schemes  A  and  B — Reduction 
with  chromosome  tetrads — Numerical  reduction  without  qualitative  reduc- 
tion— Synapsis,  or  chromosome  conjugation — Relationship  of  the  synaptic 
mates — The  stage  at  which  conjugation  occurs — The  nature  of  the  synaptic 
union — Chromomeres — Other  opinions  on  the  heterotypic  prophase- 
Bibliography  11. 

CHAPTER  XII 
Fertilization 273 

Fertilization  in  animals — The  gametes — The  fusion  of  the  gametes — 
Fertilization  in  protozoa — The  physiology  of  fertilization — Fertilization 
in  plants — Algae — Fungi — Bryophytes  and  pteridophytes — Gymnosperms — 
Angiosperms — Chromosome  behavior — Endosperm — Bibliography  12. 

CHAPTER  XIII 

Apogamy,  Apospory,  and  Parthenogenesis 311 

Apogamy — Apospory — Parthenogenesis  in  animals — Bibliography  13. 

CHAPTER  XIV 

The  Role  of  the  Cell  Organs  in  Heredity 323 

The  law  of  genetic  continuity— The  role  of  the  nucleus— The  promorphology 
of  the  ovum — Plastid  inheritance — Aleurone  inheritance — General  con- 
clusions. 


Mendelism  and  Mutation 


CONTENTS  xm 

CHAPTER   XV 

Pi 

Mendelism— A    typical    case   of    Mendelian   inheritance     The   cytological 
basis    of    Mendelism— Mutation— Mutations    accompanied     by    cl 
in  chromosome  number— Bearing  on  the  origin  of  species  and  varieties 
Mutations  accompanied  by  no  change  in  chromosome  number     <  lonclusion. 


:;.-, ! 


CHAPTER   XVI 

Sex 

Experimental  evidence  for  sex-determination — Sex-chromosomes  Sex- 
chromosomes  and  Mendelism— Experiment :il  alteration  of  the  Bex  ratio- 
Metabolic  theories  of  sex — General  discussion. 

CHAPTER  XVII 

Linkage 

A  typical  case  of  linkage— Sex-linkage— Non-disjunction— Linkage  groups 
The  chiasmatype  theory— Application  of  the  chiasmatype  theory   to  the 
problem  of  linkage — General  discussion— Other  theories  of  linkage     Value 
of  chromosome  theory  of  heredity. 

CHAPTER  XVIII 

Weismannism  and  Other  Theories 

Darwin's  hypothesis  of  pangenesis — DeVries's  theory  of  intracellular 
pangenesis — Nageli's  idioplasm  theory — Weismann's  theory  -Some  modern 
aspects  of  Weismannism — Non-factorial  theories — A  chemical  theory  of 
heredity — Conclusion — Bibliography  14  (for  Chapters  XTV-XVIII). 

Index .   .   .  427 


INTRODUCTION  TO  CYTOLOGY 


CHAPTER  I 

HISTORICAL  SKETCH 

The  history  of  cytology  falls  naturally  into  three  periods,  of  which 
the  first  begins  with  the  discovery  of  the  cell  by  Robert  Hooke  in  1665, 
the  second  with  the  foundation  of  the  Cell  Theory  by  Schleiden  and 
Schwann  in  1838-9,  and  the  third  with  the  important  researches  of 
Strasburger,  Hertwig,  Biitschli,  and  others  between  1870  and  1880.  In 
the  present  sketch  attention  will  be  confined  almost  entirely  to  the  first 
two  periods,  the  work  of  the  third,  or  modern,  period  being  dealt  with  in 
the  other  chapters  of  the  book. 

Prior  to  the  seventeenth  century  attempts  to  analyse  the  structure 
of  organisms  were  necessarily  unsatisfactory.  Aristotle  (384-:^22  B.C.) 
in  his  Be  Partibus  Animalium  distinguished  the  "homogeneous  parts'1 
and  the  " heterogeneous  parts,"  the  former  correspond ing  in  general  to 
what  we  classify  as  tissues  (bone,  fat,  cartilage,  flesh,  blood,  lymph, 
nerve,  membrane,  nails,  hair,  skin,  vessels,  tendon,  etc.),  and  the  lata 
being  the  larger  members  of  the  body  (head,  face,  hands,  feet,  trunk. 
etc.).  Theophrastus,  the  pupil  and  successor  of  Aristotle,  taught  in  his 
Historia  Plantarum  that  the  plant  body  is  composed  of  "sap/1  "vein* 
and  "flesh."  Aristotle's  classification  was  developed  further  by  Galen 
(131-201  A.D.)  and  by  his  followers.  Although  we  no  longer  regard  tin- 
above  components  as  elementary  parts,  but  rather  as  tissues  and  organs, 
the  ancients  may  be  pardoned  for  not  carrying  the  analysis  further,  for  they 
did  not  possess  the  necessary  instruments.  Something  was  then  known 
about  the  refraction  of  light,  but  it  was  not  until  many  centuries  later 
that  suitable  lenses  were  available.  The  first  compound  microscope  was 
brought  out  in  1590  by  J.  and  Z.  Janssen,  spectacle  makers  of  Middle- 
burg,  Holland;  and  during  the  first  part  of  the  seventeenth  century 
other  improved  models  were  designed  by  other  workei  These  instru- 
ments in  the  hands  of  men  possessed  of  scientific  curiosity  soon  Led  to 
many  significant  discoveries.  A  new  world  was  opened  to  the  eye 
science,  and  the  compound  microscope  has  since  remained  an  instru- 
ment of  extraordinary  value  in  biological  research. 

1 


>■  * < 


INTRODUCTION   TO  CYTOLOGY 

The  Discovery  of  the  Cell.— Cytology  may  be  said  to  have  begun  with 
the  discovery  of  the  cell  by  Roberl  Hooke  (1635-1703)  in  1665.  Hooke, 
who  lived  in  London  and  lias  been  described  as  a  man  of  eccentric  appear- 
ftnce  ftnd  habits,  showed  a  remarkably  varied  scientific  activity.  For  a 
tjIm.  ne  wafi  a  professor  of  geometry,  and  later  became  an  architect.  He 
rformed  many  original  experiments  in  mechanics  and  for  a  number  of 
-  curator  of  experiments  to  the  Royal  Society.  His  interest  in 
optics  led  him  to  examine  all  sorts  of  objects  with  the  compound  micro- 
[n  charcoal  and  later  in  cork  and  other  plant  tissues  he  found 
small  honeycomb-like  cavities  which  he  called  "cells."  He  had  no  dis- 
,,,,,. t  notion  of  the  cell  contents,  but  spoke  of  a  "nourishing  juice," 
which  he  interred  must  pass  through  pores  from  one  cell  to  another. 
His  many  observations  were  embodied  in  his  Micrographia  (1665),  a 
large  work  illustrated  with  83  plates.  The  chapter  containing  his  re- 
marks on  cells  is  entitled  "Of  the  schematisme  or  texture  of  cork  and 
the  cells  and  pores  of  some  other  such  frothy  bodies."  Quaint  and  crude 
as  it  now  appear-  to  us,  the  Micrographia  takes  its  place  as  the  earliest 

cytologies!  classic. 

Three  ol  her  names  even  more  prominent  in  the  early  history  of  micro- 
Bcopy  are  those  of  Malpighi,  Grew,  and  Leeuwenhoek.  Marcello  Mal- 
pighi i  1628  1694),  an  Italian  physiologist  and  professor  of  medicine  at 
Bologna,  Pisa  and  Messina,  is  best  known  for  his  important  pioneer  work 
in  anatomy  and  embryology.  Most  of  his  observations  on  plants  were 
included  in  his  -1  naiome  rUmtarum  (1675)  and  had  to  do  largely  with  the 
various  kinds  of  elements  making  up  the  body  of  the  vascular  plant. 
Malpighi  made  a  distinct  step  in  advance  in  studying  tissues  with  the 
cell  as  B  unit  ;  a  clear  fore-shadowing  of  the  Cell  Theory  is  seen  in  his 
remarks  concerning  the  importance  of  the  "utriculi"  in  the  structure  of 
the  body.  At  Pisa  Malpighi  was  associated  with  G.  A.  Borelli,  who  was 
one  of  the  first  to  use  the  microscope  on  the  tissues  of  higher  animals. 

Nehemiah  ( irew  (1641-1712)  was  an  English  physician  and  botanist. 
He  began  a  careful  study  of  plant  structure  in  1664,  and  in  1670  read  his 
firsl  in i poii ant  paper  before  the  Royal  Society.  Further  contributions 
followed  at  intervals  until  1682,  when  all  of  them  were  published  under 
the  till''  Thi  Anatomy  of  Plants.  Like  Malpighi,  an  abstract  of  whose 
first  work  on  plants  was  presented  to  the  Royal  Society  in  1671,  Grew  was 
interested  in  tissues,  and  "gave  particular  attention  to  the  combinations 
of  these  tissues  in  different  plant  organs.  He  was  strongly  impressed 
by  the  manner  in  which  the  cells,  which  he  also  called  "vesicles"  and 
"bladders,  "appeared  to  make  up  the  bulk  of  certain  tissues:"  .  .  .the  paren- 
chyma of  the  Barque,"  he  said,  "is  much  the  same  thing,  as  to  its  con- 
formation, which  the  froth  of  beer  or  eggs  is,  as  a  fluid,  or  a  piece  of  fine 
Manchet,  as  a  fixed  body'  (p.  64).  He  further  believed  the  walls  of 
the  cells  to  be  composed  of  numerous  extremely  fine  fibrils:  in  the  vessels 


HISTORICAL  SKETCH  3 

or  longitudinal  elements  these  fibrils  were  wound  in  the  form  of  a  close 
spiral,  while  the  vessels  themselves  were  bound  together  by  a  transverse 
series  of  interwoven  threads.  He  accordingly  compared  the  structure 
of  the  plant  with  that  of  a  basket,  and  with  "fine  bone-lace,  when  the 
women  are  working  it  upon  the  cushion"  (p.  121). 

Antony  van  Leeuwenhoek  (1632-1723)  of  Delft  is  remembered  for 
his  pioneer  researches  in  the  field  of  microscopy.  He  constructed  a 
number  of  simple  lenses  of  high  power,  and  with  these  he  was  able  to  see 
for  the  first  time  certain  protozoa,  bacteria,  and  other  minute  forms  of  life. 
In  the  course  of  his  investigations  he  observed  the  cells  ("globules") 
in  the  tissues  of  higher  organisms.  His  work,  in  spite  of  the  fact  that 
it  was  carried  on  without  any  definite  plan,  brought  to  light  a  number 
of  important  facts,  but  in  general  his  accomplishments  do  not  bear 
favorable  comparison  with  those  of  Grew  and  Malpighi. 

Preformation  and  Epigenesis.— After  the  death  of  Leeuwenhoek 
there  ensued  a  period  during  which  the  actual  investigation  of  the  cell 
and  the  structure  of  organisms  remained  practically  at  a  standstill.  At 
that  time,  however,  certain  speculations  were  indulged  in  which  should 
be  recorded  here,  not  because  they  can  be  regarded  as  scientific  cytology 
but  because  of  the  influence  they  exerted  upon  the  formulation  of  many 
cytological  problems  in  later  years.  These  speculations  resulted  in  the 
division  of  the  biologists  of  the  day  into  two  schools,  the  main  question 
at  issue  being  the  manner  in  which  the  embryo  develops  from  the  egg.  . 
The  two  theories  formulated  in  answer  to  this  question  have  been  called 
the  Preformation  Theory  and  the  Theory  of  Epigenesis. 

According  to  the  Preformation  Theory,  the  basis  for  which  was  laid 
in  the  seventeenth  century  works  of  Swammerdam,  Malpighi,  and 
Leeuwenhoek,  the  egg  contains  a  fully  formed  miniature  individual, 
which  simply  unfolds  and  enlarges  as  development  proceeds.  Because 
of  this  unfolding  the  theory  was  also  known  as  the  Theory  of  Evolu- 
tion, a  phrase  which  has  a  quite  different  connotation  today.  In  the 
eighteenth  century  the  preformation  idea  was  carried  to  an  absurd 
extreme  by  Bonnet  (1720-1793)  and  others,  who  argued  that  if  the  egg 
contains  the  complete  new  individual,  the  latter  must  in  turn  contain 
the  eggs  and  individuals  of  all  future  generations  successively  encased 
within  it,  like  an  infinite  series  of  boxes  one  within  another.  This  1  heory 
of  encasement  (emboitement)  was  a  logical  deduction  from  the  since 
abandoned  premise  that  everything,  including  organisms  for  all  time, 
had  been  formed  by  one  original  creation,  and  that  nothing  could  there- 
fore be  formed  anew.  The  preformationists  soon  became  separated  into 
two  groups:  the  spermists  or  animalculists,  and  the  ovists.  By  the 
former  the  new  individual  was  supposed  to  be  encased  in  the  sperma- 
tozoon, and  figures  were  actually  published  showing  a  small  human  figure, 
or  "homunculus,"  within  the  sperm  head.     The  ovists,  on  the  contrary, 


4  ISTRODUCTION  TO  CYTOLOGY 

held  that  the  individual  is  encased  in  the  egg.  A  bitter  strife  was  carried 
on  over  this  question  by  the  two  groups  of  preformationists,  and  various 
interesting  compromises  were  made.  But  all  extreme  forms  of  preforma- 
tionism  were  to  disappear  in  the  light  of  more  critical  investigations, 
which  went  far  to  support  the  opposing  Theory  of  Epigenesis. 

Two  of  the  early  champions  of  the  Theory  of  Epigenesis  were  William 
Harvey  (1578-1G67;  Exercitationes  de  Generatione  Animalium,  1651), 
and  Caspar  Friedrich  Wolff  (1733-1794;  Theoria  Generationis,  1759). 
As  the  result  of  many  careful  observations  on  the  embryogeny  of  the 
chick  Wolff  was  able  to  show  beyond  question  that  development  is 
epigenetic:  neither  egg  nor  spermatozoon  contains  a  formed  embryo; 
development  consists  not  in  a  process  of  unfolding,  but  in  "the  continual 
formation  of  new  parts  previously  non-existent  as  such';  (Wilson). 
Here  there  was  room  for  the  principle  of  true  generation,  or  "the 
production  of  heterogeneity  out  of  homogeneity."  The  Theoria  Genera- 
tionis is  to  be  regarded  as  one  of  the  really  great  contributions  to 
biological  science,  for  the  Theory  of  Epigenesis,  to  which  it  furnished 
substantial  support,  later  became  established  with  modifications  as  a 
fundamental  principle  of  embryology,  particularly  through  the  work  of 
von  Baer  in  the  nineteenth  century. 

In  commenting  on  preformation  and  epigenesis  Whitman  (1894) 
emphasizes  the  fact  that  the  tendency  of  modern  biology  has  not  been  to 
show  the  entire  falsity  of  either  or  both  of  these  views,  but  to  seek  out  the 
germs  of  truth  possessed  by  each,  and  to  relate  them  to  modern  biological 
conceptions.  'The  two  views  missed  the  mark  by  over-shots  in  contrary 
directions,"  says  Whitman.  The  one  theory  claimed  too  much  preforma- 
i  ion :  everything  was  preformed  at  the  start.  The  other  theory  claimed 
too  much  postformation:  everything  was  formed  anew.  Our  present 
position,  although  it  excludes  both  views  in  their  crude  original  form, 
involves  in  a  new  sense  both  conceptions.  When  we  say  that  the  egg  is 
organized,  possessing  an  architecture  or  mechanism  in  its  cytoplasm  or 
nucleus  which  largely  predetermines  development,  we  are  making  a 
modernized  statement  of  the  preformation  idea.  When  we  say  that  the 
parts  of  the  individual  are  in  no  way  delineated  in  the  egg,  but  are  mainly 
determined  by  external  conditions  during  the  course  of  development, 
we  are  speaking  in  terms  of  modern  epigenesis.  "The  question  is  no 
longer  whether  all  is  preformation  or  all  postformation ;  it  is  rather  this : 
How  far  is  post-formation  to  be  explained  as  the  result  of  pre-formation,  and 
how  far  as  the  result  of  external  influences?"  When  it  is  borne  in  mind, 
therefore,  that  one  of  the  outstanding  problems  of  modern  cytology  is 
that  of  identifying  the  factors  involved  in  the  development  of  an  organ- 
ized and  highly  differentiated  individual  from  an  organized  but  relatively 
undifferentiated  egg  cell,  it  is  at  once  evident  that  our  sketch  of  cyto- 
logical  history  would  be  incomplete  without  the  above  reference  to  the 
early  Theories  of  Preformation  and  Epigenesis. 


HISTORICAL  SKETCH  5 

Early    Theories     of     Cell-formation. — The    researches    of    Hooke, 

Malpighi,  and  Grew  in  the  seventeenth  century  had  shown  that  "cells/' 
or  "globules,"  are  important  structural  elements  in  organisms.  When 
attention  was  again  directed  to  such  matters  in  the  eighteenth  century 
there  was  very  soon  felt  a  need  for  a  theory  which  would  account  for  the 
origin  of  cells.  We  may  briefly  review  some  of  the  suggestions  which  were 
offered. 

One  of  the  earliest  theories  of  cell-formation  was  that  put  forward  by 
Wolff  in  the  Theoria  Generationis.  According  to  Wolff,  every  organ  is 
at  first  a  clear,  viscous  fluid  with  no  definite  structural  organization. 
In  this  fluid  cavities  (Blaschen;  Zellen)  arise  and  become  cells,  or,  by 
elongation,  vessels.  These  may  later  be  thickened  by  deposits  from  the 
"solidescible'  nutritive  fluid.  The  cavities,  or  cells,  are  not  to  be 
regarded  as  independent  entities;  organization  is  not  effected  by  them, 
but  they  are  rather  the  passive  results  of  an  organizing  force  (vis  essen- 
tialis)  inherent  in  the  living  mass.  Three  important  points  in  Wolff's 
theory  should  be  noted  because  of  the  relation  they  bear  to  subsequent 
conceptions  of  the  role  of  cells:  the  spontaneous  origin  of  the  cell,  the 
organization  of  parts  by  differentiation  in  a  homogeneous  living  mass, 
and  the  passive  role  of  the  cell  in  this  organizing  process.  This  theory 
was  adopted  in  1801  by  C.  F.  Mirbel  (1776-1854),  who  further  believed 
that  the  cells  communicate  through  pores  in  their  walls. 

K.  Sprengel  (1766-1833)  stated  that  cells  originate  in  the  contents 
of  other  cells  as  granules  or  vesicles  which  absorb  water  and  enlarge. 
Sprengel's  observations  seem  to  have  been  very  poorly  made,  for  he 
evidently  mistook  starch  grains  for  the  "vesicles"  which  were  supposed 
to  grow  into  new  cells.  But  Sprengel's  theory  was  upheld  by  L.  ('. 
Treviranus  (1779-1864)  in  a  work  appearing  in  1806,  and  both  men  fought 
many  years  for  its  support.  Kieser  (1812)  further  developed  the  theory 
that  granules  in  the  latex  are  "cell  germs"  which  later  hatch  in  the  inter- 
cellular spaces  to  form  new  cells. 

With  a  much  clearer  understanding  of  the  nature  of  the  problem- 
involved  a  number  of  excellent  observations  were  made  by  J.  J.  Bern- 
hardi  in  1805,  by  H.  F.  Link  and  K.  A.  Rudolphi  in  1807,  and  by  J.  J.  P. 
Moldenhawer  in  1812.  It  is  to  be  regretted  that  the  deserved  attention 
was  not  given  to  their  views,  for  they  promised  to  lead  in  the  right 
direction. 

A  number  of  years  later  Mirbel,  in  a  work  on  Marchantia  (1831-1833), 
distinguished  three  modes  of  cell-formation:  (1)  the  formation  of  cells  on 
the  surface  of  other  cells,  (2)  the  formation  of  cells  within  older  cells,  and 
(3)  the  formation  of  cells  between  older  cells.  The  first  mode  apparently 
represented  the  budding  of  the  germ  tube  arising  from  the  spore,  while  the 
second  and  third  modes  were  formulated  as  the  result  of  a  misinterpreta- 
tion of  the  process  of  cell-multiplication  in  growing  gemma?. 


6  INTRODUCTION   TO  CYTOLOGY 

Hugo  von  Mohl  (1805-1872),  in  spite  of  his  many  valuable  observa- 
tions on  the  growth  of  algae,  in  1835  agreed  essentially  with  Mirbel.  He 
made  a  step  in  advance,  however,  when  he  described  carefully  for  the 
first  time  the  division  of  a  cell.  We  shall  see  further  on  that  von  Mohl's 
later  researches  contributed  largely  to  the  upbuilding  of  an  adequate 
theory  of  the  cell. 

F.  J.  F.  Meyen  (1804-1840)  held  that  there  are  three  fundamental 
forms  of  elementary  organs:  cells,  spiral  tubes,  and  sap  vessels.  He 
noted  the  wide  occurrence  of  cell-division  but  did  not  describe  the  process 
in  detail.  Meyen  apparently  made  the  first  attempt  to  distinguish  cell- 
division  from  the  free  cell-formation  described  by  previous  workers.  It 
has  been  pointed  out  by  Sachs  that  if  this  short  step  had  been  clearly 
taken  earlier  the  peculiar  theory  of  cell-formation  later  developed  by 
Schleiden  would  have  been  impossible.  Von  Mohl  also  had  made  obser- 
vations ruling  out  Schleiden's  idea,  but  his  excessive  caution  prevented 
him  from  making  a  decisive  statement  on  the  subject.  H.  J.  Dutrochet 
(1776-1847)  in  1837  described  the  body  as  being  composed  of  solids  and 
fluids,  the  former  being  aggregations  of  cells  of  a  certain  degree  of  firmness, 
and  the  latter,  such  as  blood,  being  made  up  of  cells  freely  floating.  He 
believed  that  although  the  cell  contents  may  be  more  or  less  solid,  the 
highest  degree  of  vitality  is  compatible  only  with  the  liquid  condition. 
He  further  recognized  muscle  fibers  as  elongated  cells. 

To  all  the  above  workers  the  important  elementary  unit  was  the 
"globule."  It  was  customary  to  refer  to  this  conception  as  the  Globular 
Theory,  in  contradistinction  to  the  curious  and  fanciful  Fiber  Theory 
put  forth  by  Halle*  (1708-1777)  many  years  before  (1757),  according  to 
which  the  organism  is  made  up  of  slender  fibers  cemented  together 
by  "organized  concrete."  For  some  the  term  "globule"  stood  for  the 
granules  seen  in  the  cell  contents,  whereas  for  others  it  meant  the  cell 
itself.  As  observations  multiplied  and  ideas  became  more  definite  the 
Cell  Theory  of  Schleiden  and  Schwann  was  more  and  more  distinctly  fore- 
shadowed. Before  turning  to  the  Cell  Theory,  however,  we  must  notice 
briefly  a  few  observations  which  had  been  made  on  the  cell  contents. 

Early  Observations  on  the  Cell  Contents. — Although  the  true  nature 
and  significance  of  the  contents  of  cells  were  not  recognized  until  many 
years  later,  a  number  of  early  investigators  had  seen  protoplasm  and  had 
been  impressed  by  certain  of  its  activities.  As  early  as  1772  Corti,  and  a 
few  3rears  later  Fontana  (1781)  saw  the  rotation  of  the  "sap':  in  the 
Characese  and  other  plants.  After  being  long  forgotten  these  facts 
were  rediscovered  by  L.  C.  Treviranus  (1811)  and  G.  B.  Amici  (1819), 
whereupon  Horkel,  an  uncle  of  Schleiden,  called  attention  to  the  earlier 
work  of  Corti.  Protoplasmic  circulation  of  the  more  complex  type  was 
discovered  in  the  stamen  hairs  of  Tradescantia  by  Robert  Brown  in  1831, 
and  other  workers,  especially  Meyen,  soon  added  other  cases. 


HISTORICAL  SKETCH  7 

During  the  first  third  of  the  nineteenth  century  do  name  is  of  greater 
interest  to  cytologists  than  that  of  Robert  Brown  (1773  L858).  Al- 
though he  is  famous  chiefly  for  his  great  taxonomic  monographs  and  ln- 
morphological  work,  he  is  known  in  cytology  as  the  man  who  is  usually 
given  the  credit  for  the  discovery  of  the  nucleus,  which  he  announced 
in  1831.  Although  it  was  Brown  who  was  impressed  by  the  probable 
importance  of  the  nucleus,  and  who  concluded  in  1833  that  it  is  a  normal 
cell  element,  certain  other  observers,  notably  Fontana,  who  described  a 
nucleus  in  1781,  and  Meyen,  who  saw  it  in  Spirogyra  in  1826,  should 
share  the  honor  for  its  discovery.  The  phenomenon  which  has  since 
been  known  as  "Brownian  movement"  was  seen  by  Brown  in  1827. 

The  first  period  in  the  development  of  our  subject  is  seen  to  have 
been  one  in  which  there  was  a  tendency  to  indulge  in  speculation  to  an 
extent  quite  unwarranted  by  the  facts  at  hand.  As  we  have  already 
pointed  out,  however,  this  speculation  was  of  considerable  importance  to 
us,  in  that  it  had  to  do  with  questions  which  later  became  central  prob- 
lems of  cytology.  Carefully  made  observations  were  meanwhile  in- 
creasing in  number  and  varieyt,  and  the  time  eventually  became  ripe 
for  the  formulation  of  a  theory  which  would  correlate  these  data  and  give 
a  definite  trend  to  cytological  investigations.  Such  a  theory  was  soon 
forthcoming. 

The  Foundation  of  the  Cell  Theory. — The  year  1838  marks  an  epoch 
in  the  history  of  biology.  In  this  and  the  following  year  Schleiden  and 
Schwann  founded  the  Cell  Theory,  which,  in  view  of  its  enormous  in- 
fluence upon  all  branches  of  biological  science,  may  be  regarded  as  second 
in  importance  only  to  the  Theory  of  Evolution.  We  have  seen  that  cells 
had  been  observed  by  various  workers  during  many  years,  and  had  been 
recognized  as  being  constantly  present  in  the  bodies  of  living  organism-. 
but  it  remained  for  Schleiden  and  especially  Schwann  to  formulate  a 
comprehensive  theory  embracing  the  known  facts  and  affording  a  stall- 
ing point  for  further  researches. 

The  Cell  Theory  stated  primarily  that  the  body  is  compos,,!  entirely  of 
cells  and  their  products,  the  cell  being  the  unit  of  structure  and  function 
and  the  primary  agent  of  organization.  Subsidiary  to  this  was  Schleiden 's 
theory  of  cell-formation,  which  should  not  be  confused  with  the  main 
thesis  just  stated. 

Matthias  Jakob  Schleiden  (1804-1881)  is  one  of  the  most  prominent 
and  interesting  characters  in  botanical  history.  He  studied  law  at 
Heidelberg,  medicine  at  Gottingen,  and  botany  at  Berlin,  where  he  met 
Schwann  and  Robert  Brown.  The  association  of  these  men  undoubtedly 
meant  much  to  the  future  of  botany  and  zoology.  Eventually  Schleiden 
became  Professor  of  Botany  at  Jena,  where  he  remained  for  23  year-. 
Schleiden  was  famous  not  merely  because  of  his  own  work,  but  chiefly  as 
the  result  of  the  tremendous  impetus  which   he  gave  to  investigation. 


8  INTRODUCTION  TO  CYTOLOGY 

He  sought  to  place  botany  on  a  scientific  footing  equal  to  that  of  physics 
and  chemistry,  and  insisted  upon  accurate  observation  and  developmental 
studies  as  the  basis  of  morphology.  Sachs  says:  "Endowed  with  some- 
what too  great  love  of  combat,  and  armed  with  a  pen  regardless  of  the 
wounds  it  inflicted,  ready  to  strike  at  any  moment,  and  very  prone  to 
exaggeration,  Schleiden  was  just  the  man  needed  in  the  state  in  which 
botany  then  was." 

Theodor  Schwann  (1810-1882)  was  associated  as  a  student  with 
Johannes  M tiller,  the  great  physiologist,  first  at  Wtirzburg  and  later  at 
Berlin.  It  was  in  the  latter  place  that  he  put  forth  his  statement  of  the 
( 'ell  Theory.  Immediately  afterward  he  went  to  Louvain,  where  he  was 
a  professor  for  nine  years,  and  later  transferred  to  Liege.  In  disposition 
he  contrasted  strongly  with  Schleiden,  being  described  as  "gentle  and 

pacific." 

It  is  said  that  Schleiden,  while  dining  with  Schwann,  discussed  with 
him  some  of  his  ideas  regarding  cells  in  plants,  which  he  had  been  studying 
in  his  laboratory.  Schwann  had  been  making  similar  observations  on 
animals,  and  after  the  meal  the  two  went  to  Schwann's  laboratory,  where 
they  came  to  the  conclusion  that  cells  are  fundamentally  alike  in  both 
kingdoms.  Schleiden's  treatise  on  the  subject,  Beitrdge  zur  Phytogenesis, 
appeared  in  1838  and  dealt  mainly  with  the  origin  of  cells.  Robert  Brown 
had  recently  discovered  the  nucleus,  and  about  it  Schleiden  built  up  his 
theory  of  "free  cell-formation,"  which  was  essentially  as  follows:  In  the 
general  cell  contents  or  mother  liquor  ("  cytoblastema  ")  there  are  formed, 
by  a  process  of  condensation,  certain  small  granules  (later  called  'nu- 
cleoli" by  Schwann).  Around  these  many  other  granules  accumulate, 
thus  forming  nuclei  ("cytoblasts").  Then,  "as  soon  as  the  cytoblasts 
have  attained  their  full  size,  a  delicate  transparent  vesicle  appears  upon 
their  surface."  This  vesicle  in  each  case  enlarges  and  forms  a  new  cell, 
and,  since  it  arise?  upon  the  surface  of  the  cytoblast  (nucleus),  "the 
cytoblast  can  never  lie  free  in  the  interior  of  the  cell,  but  is  always  en- 
closed [i.e.,  imbedded]  in  the  cell  wall  .  .  .'  Schleiden  thus  regarded 
new  cell-formation  as  endogenous  ("cells  within  cells")  rather  than  the 
result  of  cell-division.  With  respect  to  the  main  proposition  of  the  Cell 
Theory  he  says  in  the  opening  paragraphs:  "...  every  plant  developed 
in  any  higher  degree,  is  an  aggregate  of  fully  individualized,  independent, 
separate  beings,  even  the  cells  themselves.  Each  cell  leads  a  double 
life:  an  independent  one,  pertaining  to  its  own  development  alone; 
and  another  incidental,  in  so  far  as  it  has  become  an  integral  part  of  a 
plant.  It  is,  however,  easy  to  perceive  that  the  vital  process  of  the  in- 
dividual cells  must  form  the  first,  absolutely  indispensable  fundamental 
basis,  both  as  regards  vegetable  physiology  and  comparative  physiology 

in  general;   .    .    . ' 

Schleiden  shared  the  results  of  his  observations,  including  his  errors, 


HISTORICAL  SKETCH  0 

with  Schwann,  who  was  the  one  to  formulate  the  Cell  Theory  in  a  com- 
prehensive manner.  Schwann  announced  the  theory  in  concise  form  in 
1838,  and  in  1839  published  a  very  full  account  under  the  title  "Afifcro- 
skopische  Untersuchungen  ilber  die  Uebereinstimmung  in  der  Struktur  und 
dem  Wachslhum  der  Thieve  und  Pflanzen."  He  says:  "The  elementary 
parts  of  all  tissues  are  formed  of  cells  in  an  analogous,  though  very  di ver- 
sified manner,  so  that  it  may  be  asserted  that  there  is  one  universal  prin- 
ciple of  development  for  the  elementary  parts  of  organisms,  however  different 
and  that  this  principle  is  the  formation  of  cells.,y  And  further:  "The 
development  of  the  proposition  that  there  exists  one  general  principle 
for  the  formation  of  all  organic  productions,  and  that  this  principle  i- 
the  formation  of  cells,  as  well  as  the  conclusions  which  may  be  drawn  from 
this  proposition,  may  be  comprised  under  the  term  Cell  Theory  .  .  .  ' 
".  .  .  all  organized  bodies  are  composed  of  essentially  similar  parts, 
namely,  of  cells   .    .    .  " 

Elaboration  of  the  Cell  Theory. — The  Cell  Theory  at  once  became 
established  as  one  of  the  main  foundation  stones  of  biological  research, 
but  it  underwent  considerable  modification  as  investigations  proceeded. 
The  main  thesis,  that  the  body  is  composed  of  cells  and  their  product-, 
remained,  but  other  ideas  associated  with  this  in  the  minds  of  Schleiden 
and  Schwann,  particularly  that  concerning  free  cell-formation,  were 
superseded.  Soon  after  the  formulation  of  the  Cell  Theory  its  elabora- 
tion was  begun  by  Unger,  von  Mohl,  and  Nageli,  who  based  their  con- 
clusions on  observations  of  a  very  high  order.  Franz  Unger  ( 1800-1 87(  >  . 
in  two  works  appearing  in  1844  on  vegetable  growing  points  and  tin- 
growth  of  internodes,  argued  for  the  origin  of  cells  by  division.  Yon 
Mohl,  in  two  treatises  (1835,  1844),  maintained  that  there  are  two  meth- 
ods of  cell-formation:  by  division  and  by  the  formation  of  cells  within 
cells^  He  thought  the  "primordial  utricle'1  (protoplast)  must  be  ab- 
sorbed to  make  way  for  the  two  new  ones,  or,  less  probably,  the  old  one 
must  divide  into  two.  Like  Schleiden,  he  thought  the  nucleus  must  be 
incorporated  in  the  cell  wall,  but  later  (1846)  concluded  that  it  lies  in 
the  primordial  utricle.  It  was  in  his  paper  of  1846  that  von  Mohl  in- 
troduced the  term  "protoplasm"  in  its  present  sense. 

Carl  von  Naegli  (1807-1891)  in  1844  produced  an  exhaustive  treatise 
on  the  nucleus,  cell-formation,  and  growth.  In  algffi  and  the  micro- 
sporocytes  of  angiosperms  he  clearly  showed  that  cells  multiply  by 
division,  and  Schleiden  was  forced  to  admit  that  this  might  be  "a  second 
kind  of  cell-formation."  The  continuation  of  Naegli's  researches  in 
1846  completely  overthrew  Schleiden's  conception  of  free  cell-formation, 
establishing  the  significant  fact  that  all  vegetative  cell-formation  is  by 
cell-division.  Many  similar  observations  had  been  made  by  Unger  and 
von  Mohl,  but  Nageli  elaborated  a  broad  theory  which  took  into  account 
all  of  the  data  at  hand.     He  distinctly  defined  cell-division  and  free 


10  INTRODUCTION  TO  CYTOLOGY 

cell-formation,  and  showed  that  what  had  been  taken  for  the  latter  was 
only  a  special  case  of  the  former.  Nageli's  conclusions  were  supported 
by  new  evidence  furnished  by  other  investigators,  who  further  demon- 
strated that  not  only  vegetative  cells  but  also  those  reproductive  cells 
(in  thallophytes)  which  Niigeli  thought  in  some  cases  might  be  formed 
freely,  originate  by  a  modified  process  of  cell-division.  It  was  now  clear 
that  cells  arise  only  from  preexisting  cells,  a  conception  which  had  been 
emphasized  by  Remak  (1841)  and  which  Virchow  (1855)  expressed  in  the 
dictum  uomnis  cell  (da  e  cellula." 

( ) pinions  concerning  the  origin  of  the  nucleus  and  its  role  in  cell- 
division  varied  greatly  among  these  workers,  reliable  observations  being 
as  yet  insufficient  to  allow  the  formulation  of  any  definite  conclusion. 
In  1841  Henle  believed  with  Schleiden  and  Schwann  that  the  nucleus  was 
formed  by  the  aggregation  of  " elementary  granules,"  and  that  it  was  not 
constantly  present.  Goodsir  looked  upon  the  nucleus  as  the  reproduc- 
tive organ  of  the  cell.  Yon  Kolliker  in  1845  asserted  that  nuclear  divi- 
sion precedes  the  division  of  the  cell,  and  Remak,  as  a  result  of  his 
observations  on  blood  cells  in  the  chick  embryo,  formulated  a  definite 
theory  of  cell-division  (1841,  1858).  He  believed  cell-division  to  be  a 
''centrifugal''  process:  the  nucleolus,  nucleus,  cytoplasm,  and  cell  mem- 
brane were  supposed  to  divide  in  turn  by  simple  constriction.  Just  such 
a  process,  though  evidently  very  exceptional,  has  been  observed  at  a 
more  recent  date  by  Conklin  (1903).  In  describing  a  case  of  nuclear 
division  Wilhelm  Hofmeister  (1824-1877)  stated  that  the  membrane  of 
the  nucleus  dissolved,  the  nuclear  material  then  separating  into  two 
masses  around  which  new  membranes  were  formed  (1848,  1849).  It  was 
generally  believed,  however,  that  the  origin  of  nuclei  by  division  was 
of  rare  occurrence,  and  that  ordinarily  the  nucleus  dissolved  just  before 
cell-division,  two  new  ones  forming  de  novo  in  the  daughter  cells.  Von 
Mohl  (1851),  who  in  the  main  agreed  with  Hofmeister,  wrote  as  follows: 
'  The  second  mode  of  origin  of  a  nucleus,  by  division  of  a  nucleus  already 
existing  in  the  parent-cell,  seems  to  be  much  rarer  than  the  new  produc- 
tion of  them  .  .  .  '  And  again,  "  .  .  .  it  is  possible  that  this  process 
[nuclear  division]  prevails  very  widely,  since  ...  we  know  very  little 
yet  respecting  the  origin  of  nuclei.  Naegli  thinks  that  the  process  is 
quite  similar  to  that  in  cell-division,  the  membrane  of  the  nucleus  form- 
ing a  partition,  and  the  two  portions  separating  in  the  form  of  two  dis- 
tinct cells." 

It  was  not  until  many  years  later,  in  connection  with  researches  upon 
fertilization  and  embryogeny,  that  the  behavior  of  the  nucleus  in  cell- 
division  became  known  in  detail,  and  its  probable  significance  pointed 
out.  In  1879  Eduard  Strasburger  (1844-1912)  announced  definitely 
that  nuclei  arise  only  from  preexisting  nuclei.  W.  Flemming  was  led 
to  the  same  conclusion  by  his  studies  on  animal  cells,  and  expressed 
it  in  the  dictum  "omnis  nucleus  e  nucleo"  (1882).     (See  footnote,  p.  143.) 


HISTORICAL  SKETCH  11 

The  Protoplasm  Doctrine.- -The  Cell  Theory  and  all  of  its  corollaries 
were  placed  in  a  new  lighl  with  the  development  of  a  more  adequate 
conception  of  the  significance  of  protoplasm.  To  its  discoverers  the 
cell  meant  nothing  more  than  the  wall  surrounding  a  cavity:  they  spoke 
only  in  the  vaguest  terms  of  the  " juices"  present  in  cellular  structures. 
The  founders  of  the  Cell  Theory  hold  a  position  but  little  in  advance  of 
this;  they  observed  the  cell  contents  but  regarded  them  as  of  relatively 
slight  importance.  Even  those  who  had  been  impressed  by  the  phe- 
nomenon of  protoplasmic  streaming  wore  not  aware  of  the  significance  of 
the  substance  before  their  eyes. 

Felix  Dujardin  (1801-1860)  in  1835  described  the  "sarcode"  of  the 
lower  animals  as  a  substance  having  the  properties  of  life.  Von  Mohl 
had  seen  a  similar  substance  in  plant  cells,  and  in  1846,  as  noted  above. 
he  called  it  "Schleim,"  or  "Protoplasma,"  the  latter  term  having  been 
used  shortly  before  by  Purkinje  in  a  somewhat  different  sense.  Nageli 
and  A.  Payen  (1795-1871)  in  1846  recognized  the  importance  of  proto- 
plasm as  the  vehicle  of  the  vital  activity  of  the  cell;  and  Alexander  Braun 
(1805-1877)  in  1850  pointed  out  that  swarm  spores,  which  are  cells,  con- 
sist of  naked  protoplasm.  An  important  point  was  reached  when  Payen 
(1846)  and  Ferdinand  Cohn  (1850)  concluded  that  the  "sarcode"  of 
the  animal  and  the  " protoplasm'  of  the  plant  are  essentially  similar 
substances.     In  the  words  of  Cohn : 

"The  protoplasm  of  the  botanist,  and  the  contractile  substance  and  sarcode 
of  the  zoologist,  must  be,  if  not  identical,  yet  in  a  high  degree  analogous  sub- 
stances. Hence,  from  this  point  of  view,  the  difference  between  animals  and 
plants  consists  in  this;  that,  in  the  latter,  the  contractile  substance,  as  a  primordial 
utricle,  is  enclosed  within  an  inert  cellulose  membrane,  which  permits  it  only  to 
exhibit  an  internal  motion,  expressed  by  the  phenomena  of  rotation  and  circula- 
tion, while,  in  the  former,  it  is  not  so  enclosed.  The  protoplasm  in  the  form 
of  the  primordial  utricle  is,  as  it  were,  the  animal  element  in  the  plant,  but  which 
is  imprisoned,  and  only  becomes  free  in  the  animal;  or,  to  strip  off  the  metaphor 
which  obscures  simple  thought,  the  energy  of  organic  vitality  which  is  manifested 
in  movement  is  especially  exhibited  by  a  nitrogenous  contractile  substance,  which 
in  plants  is  limited  and  fettered  by  an  inert  membrane,  in  animals  not   so." 

Protoplasm  was  now  studied  more  intensively  than  ever.  11.  A. 
dv  Bary  (1831-1888),  working  on  myxomycetes  and  other  plant  forms. 
and  Max  Schultze  (1825-1874),  investigating  animal  cells, demonstrated 
the  correctness  of  Cohn's  view.  The  work  of  Schultze  was  especially 
important  in  that  it  firmly  established  in  1861  the  Protoplasm  Doctrine, 
namely,  that  the  units  of  organization  are  masses  of  protoplasm,  and  that 
this  substance  is  essentially  similar  in  all  living  organisms.  The  cell, 
according  to  Schultze,  is  "a  mass  of  protoplasm  containing  a  nucleus. 
both  nucleus  and  protoplasm  arising  through  the  division  of  the  corres- 
ponding elements  of  a  preexisting  cell."     The  cell  wall,  upon  which  the 


12  INTRODUCTION  TO  CYTOLOGY 

early  workers  had  focussed  their  attention,  turned  out  to  be  of  secondary 
importance.  The  cell  was  thus  seen  to  be  primarily  the  organized 
protoplasmic  mass,  to  which  Hanstein  in  1880  applied  the  convenient 
term  protoplast. 

Extensive  studies  on  the  physical  nature  of  protoplasm  were  soon 
undertaken  by  Kiihne  (1864),  Cienkowski  (1863),  and  de  Bary  (1859, 
L864);  and  there  later  followed  the  well-known  structural  theories  of 
Klein,  Flemming,  Altman,  and  Btitschli.     (See  Chapter  III.) 

Yon  Mohl  as  early  as  1837  held  that  the  plastid  is  a  protoplasmic 
bod}-.  The  classic  researches  of  Nageli  (1858,  1863)  on  plastids  and 
starch  grains  laid  the  foundation  for  our  knowledge  of  these  bodies,  which 
was  greatly  extended  in  later  years  by  Meyer  (1881,  1883,  etc.)  and 
Schimper  (1880,  etc.).     (See  Chapter  VI.) 

It  would  be  difficult  to  overestimate  the  value,  both  practical  and 
theoretical,  of  the  Protoplasm  Doctrine,  for  its  establishment  has  not 
only  led  to  knowledge  by  which  the  conditions  of  life  have  been  materially 
improved,  but  has  also  been  an  important  factor  in  assisting  man  to  a 
modern,  rational  outlook  on  organic  nature,  in  which  he  has  learned  to 
include  himself.  It  is  not  too  much  to  say  that  the  identification  of 
protoplasm  as  the  material  substratum  of  the  life  processes  was  one  of 
the  most  significant  events  of  the  nineteenth  century.  The  doctrine 
was  furnished  with  a  popular  expression  by  Huxley  in  his  well-known 
essay,  The  Physical  Basis  of  Life  (1868). 

The  New  Conception  of  the  Cell. — The  conception  of  the  cell  had  now 
developed  into  something  quite  different  from  what  it  had  been  in  the 
minds  of  the  founders  of  the  Cell  Theory.  The  cell  was  now  recognized 
as  a  protoplasmic  unit,  and  the  ideas  of  these  men  concerning  the  origin 
and  multiplication  of  cells  had  been  overthrown.  Future  researches 
were  to  show  more  clearly  the  importance  of  the  cell  in  connection  with 
development  and  inheritance,  and  certain  limits  were  to  be  set  to  the 
conception  of  the  cell  as  a  unit  of  function  and  organization.  To  Schlei- 
den  and  Schwann  the  multicellular  plant  or  animal  appeared  as  little 
more  than  a  cell  aggregate,  the  cells  being  the  primary  individualities; 
the  organism  was  looked  upon  as  something  completely  dependent  upon 
their  varied  activities  for  all  its  phenomena.  "The  cause  of  nutrition 
and  growth,"  said  Schwann,  "  resides  not  in  the  organism  as  a  whole, 
but  in  the  separate  elementary  parts — the  cells."  This  elementalistic 
conception  of  the  organism  as  an  aggregate  of  independent  vital  units 
governing  the  activities  of  the  whole  dominated  biology  for  many  years, 
notwithstanding  its  severe  criticism  by  Sachs,  de  Bary,  and  many  other 
later  writers  who  pointed  out  that,  owing  to  the  high  degree  of  physio- 
logical differentiation  among  the  various  tissues  and  organs,  the  cell 
cannot  be  regarded  merely  as  an  independent  unit,  but  as  an  integral 
part  of  a  higher  individual  organization,  and  that  as  such  the  exercise 


HISTORICAL   SKETCH  13 

of  its  functions  must  be  governed  to  a  considerable  extenl  by  the  organ- 
ism as  a  whole  (Wager).     Such  divergence  of  opinion  led  to  much  dis- 
cussion  over  the   question   of   organic  individuality,  which  remain- 
one  of  the  important  problems  of  modern  biology. 

But  in  spite  of  all  these  changes  we  should  not  forget  the  great  service 
rendered  by  Schleiden  and  Schwann  in  the  formulation  of  the  Cell  Theory. 
Huxley  (1853)  estimated  the  value  of  their  contribution  in  the  following 
lines: 

"Doubtless  the  truer  a  theory  is — the  more  appropriate  the  colligating 
conception — the  better  will  it  serve  its  mnemonic  purpose,  but  its  absolute 
truth  is  neither  necessary  to  its  usefulness,  nor  indeed  in  any  way  cognizable  by 
the  human  faculties.  Now  it  appears  to  us  that  Schwann  and  Schleiden  have 
performed  precisely  this  service  to  the  biological  sciences.  At  a  time  when  the 
researches  of  innumerable  guideless  investigators,  called  into  existence  by  the 
tempting  facilities  offered  by  the  improvement  of  microscopes,  threatened  to 
swamp  science  in  minutiaB,  and  to  render  the  noble  calling  of  the  physiologist 
identical  with  that  of  the  'putter-up'  of  preparations,  they  stepped  forward  with 
the  cell  theory  as  a  colligation  of  the  facts.  To  the  investigator,  they  afforded 
a  clear  basis  and  a  starting  point  for  his  inquiries;  for  the  student,  they  grouped 
immense  masses  of  details  in  a  clear  and  perspicuous  manner.  Let  us  not  be 
ungrateful  for  what  they  brought.  If  not  absolutely  true,  it  was  the  truest 
thing  that  had  been  done  in  biology  for  half  a  century." 

Fertilization  and  Embryogeny.— In  Plants. — Although  it  was  known 
to  the  ancients  that  there  is  in  plants  something  analogous  to  the  sexual 
reproduction  seen  in  animals,  ideas  of  the  organs  and  processes  involved 
were  very  vague.  Like  Grew  and  others  in  the  seventeenth  century,  the 
botanists  of  antiquity  were  aware  of  the  fact  that  the  pollen  in  some  way 
influences  the  development  of  the  ovary  into  a  fruit  with  seeds.  Definite 
proof  that  the  stamens  are  (to  speak  somewhat  loosely)  the  male  organs 
was  furnished  in  the  well-known  experiments  of  R.  J.  Camerarius  ( 1691). 
But  in  spite  of  the  excellent  work  of  J.  G.  Koelreuter  (1761),  C.  K. 
Sprengel  (1793),  and  K.  F.  Gaertner  (1849),  all  of  whom  proved  the 
correctness  of  this  conclusion,  the  idea  of  sexuality  in  plants  was  vigor- 
ously combatted  in  certain  quarters  for  many  years. 

An  important  step  in  advance  was  made  when  (!.  B.  Amici  (1830) 
followed  the  growth  of  the  pollen  tube  from  the  pollen  main  on  the  Btigma 
down  to  the  ovule.  Schleiden  (1837)  and  Schacht  (1850,1858  took  up 
the  study  and  made  a  curious  misinterpretation:  they  regarded  the  ovule 
as  merely  a  place  of  incubation  for  the  end  of  the  pollen  tube,  which  they 
supposed  to  enter  the  ovule  and  enlarge  to  form  the  embryo  directly. 
The  work  of  Amici  (1842),  Tulasne  I  1849),  and  others  showed  the  falsity 
of  this  notion,  but  an  acrimonious  discussion  raged  about  the  subject 
for  a  number  of  years,  Schleiden  (1842.  L844)  using  the  most  vigorous 
language  in  support  of  his  position.     After  Hofmeister  (1849)  had  fol- 


14  INTRODUCTION  TO  CYTOLOGY 

lowed  the  process  with  his  characteristic  thoroughness  there  could  remain 
no  doubt  concerning  the  error  of  Schleiden  and  Schacht.  Hofmeister 
clearly  demonstrated  that  the  embryo  arises,  as  Amici  contended,  not 
from  the  end  of  the  pollen  tube,  but  from  an  egg  contained  in  the  ovule, 
the  egg  being  stimulated  to  development  by  the  pollen  tube.  He  was 
wrong,  however,  in  supposing  that  the  tube  did  not  open,  but  that  a 
fertilizing  substance  diffused  through  its  wall. 

It  was  in  the  algae  that  the  union  of  the  sperm  cell  with  the  egg  cell 
(the  act  of  fertilization)  was  first  seen  in  the  case  of  plants.  In  1853 
Thuret  saw  spermatozoids  attach  themselves  to  the  egg  of  Fucus,  and  in 
1854  he  showed  that  they  are  necessary  to  its  development.  The  actual 
entrance  of  the  spermatozoid  into  the  egg  was  first  observed  in  1856  by 
Nathanael  Pringsheim  (1824-1894)  in  (Edogonium.  The  fusion  of  the 
parental  nuclei  was  seen  by  Strasburger  (1877)  in  Spirogyra,  but  he 
thought  they  thereupon  dissolved.  This  error  was  corrected  shortly 
afterward  by  Schmitz  (1879),  who  was  thus  the  first  to  show  clearly  that 
the  central  feature  of  the  sexual  process  in  plants  is  the  union  of  two 
parental  nuclei  to  form  the  primary  nucleus  of  the  new  individual. 

That  the  same  process  occurs  in  fertilization  in  the  higher  plants 
was  demonstrated  by  Strasburger,  who  in  1884  described  the  union  of  the 
egg  nucleus  with  a  nucleus  brought  in  by  the  pollen  tube.  In  1898  and 
1899  S.  Nawaschin  and  L.  Guignard  completed  the  story  by  describing 
the  phenomenon  of  double  fertilization,  whereby  the  second  male  nucleus 
contributed  by  the  pollen  tube  unites  with  the  two  polar  nuclei  to  form 
the  primary  endosperm  nucleus.  The  subsequent  work  of  Strasburger 
and  others  on  the  gymnosperms  and  angiosperms  greatly  cleared  up  the 
whole  matter  of  fertilization  and  embryogeny  in  these  plants.  This 
work  belongs  to  the  modern  period  of  cytology. 

In  Animals. — It  is  probable  that  the  spermatozoon  was  first  seen  in 
1677  by  Ludwig  Hamm,  a  pupil  of  Leeuwenhoek.  The  credit  for  the 
discovery,  however,  is  usually  given  to  Leeuwenhoek,  since  it  was  he  who 
brought  the  matter  to  the  attention  of  the  Royal  Society  and  pursued 
such  studies  further.  He  asserted  that  the  spermatozoa  must  penetrate 
into  the  egg,  but  it  was  thought  at  that  time  and  for  many  years  after- 
ward that  they  were  parasitic  animalcules  in  the  spermatic  liquid;  hence 
the  name  " spermatozoa." 

Although  L.  Spallanzani  (178.6)  is  usually  said  to  have  shown  by  his 
filtration  experiment  that  the  spermatozoon  is  the  fertilizing  element, 
it  is  pointed  out  by  Lillie  (1916)  that  Spallanzani  did  not  draw  the  correct 
conclusion:  he  even  denied  that  the  spermatozoon  is  the  active  element, 
holding  rather  that  the  fertilizing  power  lies  in  the  spermatic  liquid.  It 
was  Prevost  and  Dumas  who  corrected  this  mistake  and  demonstrated 
the  true  role  of  the  spermatozoon  (1824).  The  spermatozoon  was  later 
shown  by  Schweigger-Seidel  (1865)  and  La  Valette  St.  George  (1865)  to 


HISTORICAL  SKETCH  15 

be  a  complete  cell  with  its  nucleus  and  cytoplasm,  as  von  Kolliker  had 
maintained.  That  Schwann  (1839)  had  been  righl  in  considering  the 
egg  as  a  cell  was  shown  by  Gegenbaur  in  1861.  The  polar  bodies  formed 
at  the  time  the  egg  matures  are  said  to  have  been  first  seen  by  Cams 
(1824).  Biitschli  (1875)  showed  them  to  be  formed  as  the  result  of  the 
division  of  the  egg  nucleus,  and  Giard  (1877)  and  Mark  (1881)  interpreted 
them  as  abortive  eggs. 

The  penetration  of  the  spermatozoon  into  the  egg  was  not  actually 
seen  until  Newport  (1854)  observed  it  in  the  case  of  the  frog.  In  1875 
0.  Hertwig  (b.  1849)  announced  the  important  discovery  that  the  two 
nuclei  seen  fusing  in  the  fertilized  egg  are  furnished  by  the  egg  and  the 
spermatozoon — by  the  two  parents.  The  role  of  the  nucleus  in  fertiliza- 
tion was  thus  demonstrated  in  animals  only  shortly  before  it  was  in 
plants,  and  it  is  interesting  to  note  that  the  first  complete  description  of 
the  union  of  the  germ  cells  in  animals  was  given  by  H.  Fol  in  the  same 
year  (1879)  that  Schmitz  described  clearly  the  process  in  plants.  It  was 
now  evident  that  fertilization  in  both  kingdoms  consists  in  the  union  of 
two  cells  (gametes),  one  from  each  parent  (in  dioecious  forms),  and  that 
the  central  feature  of  the  process  is  the  union  of  the  two  gamete  nuclei,  the 
new  individual  therefore  deriving  half  of  its  nuclear  substance  from  each 
parent. 

Although  the  cleavage  of  the  fertilized  animal  egg  to  form  the  embryo 
had  been  seen  many  years  previously,  it  was  first  definitely  described  by 
Prevost  and  Dumas  in  1824  for  the  frog.  At  that  time  neither  the  egg  aor 
the  products  of  its  division  were  known  to  be  cells.  The  true  meaning  of 
cleavage  was  elucidated  by  M.  Barry,  who  held  that  the  blastomeres  are 
cells  and  that  their  division  is  preceded  by  the  division  of  their  nuclei. 
and  by  a  number  of  later  writers,  including  A.  von  Kolliker,  who  traced 
in  detail  the  long  series  of  changes  by  which  the  multiplying  embryonic 
cells  become  differentiated  into  the  various  tissues  and  organs.  Embry- 
ogeny  was  thus  shown  to  be  a  process  of  cell-division  and  differentiation, 
the  fertilized  egg  cell  initiating  a  series  of  divisions  giving  rise  to  all  the 
cells  of  the  body,  and  to  the  germ  cells.  The  life  cycle  was  now  recognized 
as  a  cell  cycle;  and  since  the  egg  is  the  direct  descendant  of  the  egg  of  the 
previous  generation  it  became  evident,  as  Virchow  pointed  out  in  L858, 
that  there  has  been  an  uninterrupted  series  of  cell-divisions  from  the 
beginnings  of  life  on  the  earth  in  the  remote  past  down  to  the  organisms 
in  existence  today.  The  statement  of  this  conception  is  known  as  the 
Law  of  Genetic  Continuity.     In  the  words  of  Locy  (1915) : 

"The  conception  that  there  is  unbroken  continuity  of  germinal  substance 

between  all  living  organisms,  and  that  the  egg  and  the  sperm  are  endowed  with  an 
inherited  organization  of  great  complexity,  has  become  the  basis  for  all  current 
theories  of  heredity  and  development.  So  much  is  involved  in  this  conception 
that    ...   it    has    been    designated   (Whitman)   'the  central  fact   of    modern 


16  INTRODUCTION  TO  CYTOLOGY 

biology.'  The  first  clear  expression  of  it  is  found  in  Virchow's  Cellular  Pathology , 
published  in  1858.  It  was  not,  however,  until  the  period  of  Balfour,  and  through 
the  work  of  Fol,  Van  Beneden  (chromosomes,  1883)  Boveri,  Hertwig,  and 
others,  that  the  great  importance  of  this  conception  began  to  be  appreciated, 
and  came  to  be  woven  into  the  fundamental  ideas  of  development.'' 

The  Beginning  of  the  Modern  Period  in  Cytology. — As  Wilson  (1900, 
n.  6)  points  out,  the  great  significance  of  the  many  facts  brought  to  light 
in  the  early  days  of  cytology  lies  in  the  relation  which  they  bear  to  the 
Theory  of  Evolution  and  to  the  problems  of  heredity,  though  for  many 
years  this  was  only  vaguely  realized.  Darwin,  aside  from  his  Hypothesis 
of  Pangenesis,  scarcely  mentioned  the  theories  of  the  cell;  and  not  until 
many  years  later  was  the  cell  investigated  with  reference  to  these  matters. 
Researches  on  the  origin  of  the  germ  cells,  nuclear  division,  and  fertiliza- 
tion, which  brought  the  Cell  Theory  and  the  Theory  of  Evolution  into 
intimate  association,  began  shortly  after  1870  with  the  works  of  Schneider 
(1873),  Auerbach  (1874),  Fol  (1875,  etc.),  Butschli  (1875,  etc.),  O.Hertwig 
(1875,  etc.),  van  Beneden  (1875,  etc.),  Strasburger  (1875,  etc.),  Flemming 
(1879,  etc.),  and  Boveri  (1887,  etc.).  These  men  laid  the  foundations  for 
the  work  which  has  followed;  and  their  researches,  greatly  aided  by  the 
development  of  new  refinements  in  microtechnique,  ushered  in  modern 
cytology.  A  powerful  stimulus  to  investigation  was  given  when  the 
zoologists  Hertwig,  von  Kolliker  and  Weismann,  and  the  botanist  Stras- 
burger, concluded  independently  and  almost  simultaneously  (1884-1885) 
that  the  nucleus  is  the  vehicle  of  heredity,  an  idea  which  Haeckel  had  put 
forward  as  a  speculation  in  1866.  The  announcement  of  this  conception 
led  to  an  even  more  intensive  study  of  the  nucleus  and  of  its  role  in 
heredity,  a  study  which  is  now  in  progress,  and  which,  more  than  any 
other  one  thing,  can  be  said  to  characterize  the  work  of  our  modern  period. 

Bibliography  1 

A.   Works  dealing  wholly  or  in  part  with  the  history  of  cytotogy,  and  general  works  on 
the  cell: 

Agar,  W.  E.     1920.     Cytology,  with  Special  Reference  to  the  Metazoan  Nucleus. 

London. 
Boveri,  Th.     1891.     Befruchtung.     Ergeb.  d.  Anat,  u.  Entw.  1:  386-485. 
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deduced  from  original  investigations.     Trans.  Am    Med.  Assn.  6. 
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Paris. 

Doncaster,  L.     1920.     An  Introduction  to  the  Study  of  Cytology.     London. 
Flemming,  W.     1882.     Zellsubstanz,  Kern  und  Kerntheilung.     Leipzig. 

1981-1897.     Referate  iiber  Zelle.     Ergeb.  d.  Anat.  u.  Entw.  1-7. 
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HISTORICAL  SKETCH  17 

Henneguy,  L.  F.     1896.     Lecons  sur  la  Cellule.     Paris. 

Hertwig,  O.     1893.     Die  Zelle  und  die  Gewebe.     Jena.     (Engl.  Transl.  by  11.  J. 
Campbell.) 
1900.     Die  Entwicklung  der  Biologie  im  19.     Jahrhundert.     Jena. 
Hofmeister,  W.     1867.     Die  Lehre  von  der  Pflanzenzelle. 
Huxley,    T.  H.     1853.     The   Cell   Theory.     Brit,    and    For.   Med.-Chir.   Rev.  12. 

Also  in  "Scientific  Memoirs"  1.     1898. 
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discovery  of  sex  in  plants.     Science  39:  299-319. 
Kellogg,  V.  L.     1907.     Darwinism  Today.     New  York. 

Lillie,  F.  R.     1916.     The  history  of  the  fertilization  problem.     Science  43:  39  53 
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deu.  Bot.  Ges.  21:  (66)-(134). 
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58:  561-584. 
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Mark,  E.  L.     1881.     Maturation,  fecundation,  and  segmentation  in  Limns  campes- 
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Meves,  Fr.     1896,  1898.     Referate  iiber  Zelltheilung.     Ergeb.  d.  Anat.  u.  Entw.  6,  8. 
von  Mohl,   H.     1851.     The  Vegetable  Cell.     (Engl,  transl.  by  Henfrey.) 
Osborn,  H.  F.     1894.     From  the  Greeks  to  Darwin. 
Ruckert,   J.     1893.     Die   Chromatinreduktion  bei  der  Reifung  der  Sexualzellen. 

.   Ergeb.  d.  Anat.  u.  Entw.  3:  517-583. 
von  Sachs,  J.  1875.     History  of  Botany.     (Engl,  transl.  1889.) 
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Modern  Science"  (Seward,  editor). 
Thomson,  J.  A.     1899.     The  Science  of  Life.     Chapters  9  and  10. 
Turner,  W.     1890.     The  cell  theory,  past  and  present.     Nature  43:  10-15. 
Tyson,  J.     1878.     The  Cell  Doctrine:  its  History  and  Present  State.     Phila. 
Wager,  H.     1911.     Article  on  Plants:  Cytology,  in  Encycl.  Brit,,  11th  ed. 
Waldeyer,  W.     1888.     Ueber  Karyokinese  und  ihrc  Beziehung  zu  den  Befruchl  angs- 
vorgangen.     Arch.  Mikr.  Anat.  32:  1-122.     (Engl,  transl.  in  Quar.  Jour.  Micr. 
Sci.  30:  159-281.     1889.)     (Early  cell  literature.) 
Whitman,  C.  O.     1878.     The  embryology  of  Clepsine.     Quar.  Jour.  Micr.  Sci.  18: 
215-315.     pis.  12-15.     (Early  literature  of  mitosis  and  fertilization.) 
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Wheeler,  W.  M.     1898.     Caspar  Friedrich  Wolff  and  the  Theoria  Generatiohis. 

Ibid.  1898. 
Wilson,  E.  B.     1900.     The  Cell  in  Development  and  Inheritance.     2d  ed. 
Zimmermann,    A.     1893-1894.     Sammcl-Referate    aus    dem    Gesammtgebiete    der 
Zellenlehre.     Beih.  Bot.  Centr.  3  and  4.      (Reviews  of  early  literature). 

For  other  reviews  of  early  botanical  cell  literature  Bee  Jahresberichl  Qber  die  Fort- 
schritte  der  Anatomie  und  Physiologie,  15-20.  1886-1891';  and  Neue  Polge,  Vols. 
1-13.     1892-1907. 

B.  Special  works  referred  to  in  historical  sketch: 

Amici,  G.  B.     1830.     Note  sur  le  mode  d'action  du  pollen  sur  le  stigmate.       Extrait 

d'une  lettre  de  M.  Amici  a  M.  Mirbel.)     Ann.  Sci.   Nat.  Bot.  I.  21:  329  332. 

Account  of  Amici's  work  in  Atti  della  quarta   Etiunione  degli  Bcientiati  Ltaliani 

2 


is  INTRODUCTION   TO  CYTOLOGY 

tenuta  in  Padova  nel  Settembre  del  1S42.     Padova  1843.     See  Facchini  1845. 

See  also  (liorn.  Bot.  Ital.  Anno  2. 
^ubrbach,  L.     1874.     Organologische  Studien.     Breslau. 
von   Baku,  K.  E.  L828,  1837.     Uber  Entwickelungsgeschichte  der  Thiere. 
Barry,  M.      L838-1841.     Embryological  memoirs  in  Phil.  Trans.  Roy.  Soc.  London 

128 -131.     Sec  also:  On  the  first  changes  consequent  on  fecundation  in  the  mam- 

miferous  ovum.     Rep.  Brit.  Assn.  Adv.  Sci.     1840. 
de  Baby,  H.  A.      1S02.     Uber  den  Bau  und  das  Wesen  der  Zelle.     Flora  20. 

1864.     Die  Mycetozoen.     2d  ed.  Leipzig. 
van    Beneden,   E.      L875.     La  maturation  de  l'oeuf,  la  fecondation  et  les  premieres 

phases  tin  developpement  embryonnaire  des  mamiferes  d'apres  des  recherches 

faites  chez  le  lapin.     Bull.  Acad.  Roy.  Belg.  40. 
1876.     Contribution  a  l'histoire  de  la  vesicule  germinative  et  du  premier  noyau 

embryonnaire.     Ibid.  41. 
1SN3.      Recherches  sur  la  maturation  de  l'oeuf,  la  fecondation  et  la  division  cellu- 

laire.     Arch,  de  Biol.  4. 
Bernhardi,  J.  J.     1805.     Beobachtungen  tiber  Pflanzengefasse. 
Boveri,  Th.     1887a.     Ueber  die  Befruchtung  der  Eier  von  Ascaris  megalocephala. 

Sitzungsber.  Gesell.  Morph.  Fhys.     Miinchen  3. 
18876.     Ueber  Differenzierung  der  Zellkerne  wahrend  der  Furchung  des  Eies  von 

Ascaris  megalocephala.     Anat.  Anz.  2 :  688-693. 
1887-1890.     Zellenstudien  1,  II,  III.     Jenaische  Zeitsch.  21-24. 
Braun,  Alex.     1850.     Betrachtungen  tiber  die  Erscheinung  der  Verjiingung  in  der 

Natur,  inbesondere  in  der  Lebens-  und  Bildungsgeschichte  der  Pflanze.     (English 

transl.,  Ray  Society  1853.) 
Brown,  R.     1833.     Observations  on  the  organs  and  mode  of  fecundation  in  Orchidese 

and  Asclepiadeae.     Trans.  Linn.  Soc.     (Paper  read  and  privately  printed  in  1831.) 

Also  in:  Misc.  Bot.  Works,  Ray  Society,  1866. 
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the  pollen  of  plants;  and  on  the  general  existence  of  active  molecules  in  organic 

and  inorganic  bodies.     Misc.  Bot.  Works,  Ray  Society.     (Observations  made  in 

1827.) 
BtiTsCHLi,  O.     1875a.     Vorl.  Mitteilung  uber  Untersuchungen  betreffend  die  ersten 

Entwicklungsvorgange   im   befruchteten    Ei  von    Nematoden    und   Schnecken. 

Zeit.  Wiss.  Zool.  25:  201. 
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1876.     Studien  iiber  die  ersten  Entwicklungsvorgange  der  Eizelle,  die  Zelltheilung, 

und     die     Konj  ligation     der    Inf  usorien.     Abhandl.     Senckenb.     Naturforsch. 

Gesell.  10. 
Camerarius,  R.  J.     1691.     De  Sexu  Plantarum.     1694. 
Cares,    C.     1824.     Von   den   ausseren   Lebensbedingungen   der   Weiss-    und    Kalt- 

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Corti,  B.     1772.     Observationi  misc.  sulla  Tremella  e  sulla  circolazione  del  fluid  in 

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HISTORICAL  SKETCH  |«) 

Dutbochet,  H.  J.     1837.     Mcmoires  pour  servir  &  I'histoire  anatomique  el  physio- 

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1882.     Zellsubstanz,  Kern  und  ZeUtheilung.     Leipzig. 
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1875.  Sur  le  developpement  des  Pteropodes.     Arch,  de  Zool.  4. 

1876.  Sur  les  phenomenes  de  la  division  cellulaire.     Comptes  Rend.  Acad.  Sci. 
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1S77.     Sur  le  commencement  de  l'henogenie  chez  divers  animaux.     Arch.  Sci.  Na  i 

et  Phys.  Geneve  58. 
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Gaertner,  K.  F.     1849.     Versuche  und  Beobachtungen  liber  die  Bastard zeugung. 

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1844.  Einige  Betrachtungen  liber  den  Bau  der  vegetabilische  Zelle.     Bot.  Zeit. 
2:  273-277,  289-294,  305-310,  321-326,  337-342.     pi.  2. 

1845.  Vermischte  Schriften. 

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transl.  by  Henfrey,  London,  1852.) 
Moldenhawer,  J.  J.  P.     1812.     Beitrage  zur  Anatomie  der  Pflanzen. 
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Wiss.  Bot,  1,  3.     (Engl,  transl.  by  Henfrey,  Ray  Society;  London,  1846,  1849.) 
1846.     On  the  utricular  structures  in  the  contents  of  cells.     Ray  Society,   1849. 

(Engl.  Transl.  by  Henfrey.) 
1858.     Die  Starkekorner.     Zurich. 
Nawaschin,  S.     1899.     Neue  Beobachtungen  iiber  Befruchtung  bei  Fritillaria  und 

Lilium.     Bot.  Centr.  77:  62.     (Account  in  Russian,  1898.) 
Newport,  G.     1851,  1853,  1854.     On  the  impregnation  of  the  ovum  in  the  amphi- 
bia.    Phil.  Trans.  Roy.  Soc.  London. 
Payen,  A.     1839.     Memoire  sur  l'amidon,  etc.     Paris. 

1846.     Memoire  sur  les  developpements  des  vegetaux,  etc.     Mem.  Acad.     Paris  9. 
Prevost  and  Dumas.     1824.     Nouvelle  Theorie  de  la  generation.     Ann.  Sci.  Nat,  1 : 

1,  167;  2:  100,  129. 
Pringsheim,  N.     1855.     Ueber  die  Befruchtung  und  Keimung  der  Algen  und  das 
Wesen  des  Zeugungsaktes.     Monatsber.  K.  Akad.  Wiss.  Berlin  1. 
1856.     Ueber  die  Befruchtung  der  Algen.     Ibid. 
1858.     Morphologie  der  Oedogonieen.     Jahrb.  Wiss.  Bot.  1:  11-81. 


HISTORICAL  SKETCH  21 

Remak,   R.     1841.     Ueber  Theilung  rother  Blutzellen  beim   Embryo.     Med.  Ver. 
Zeit.     Muller's  Archiv  f.  Anat.  u.  Physiol.  1858;  177-188.     pi.   s 
1852.     Ueber  extracellulare  Entstehung  thierische  Zellen  und  Qber  Vermehrung 

derselben  durch  Theilung.     Muller's  Archiv  f.  Anat.  u.  Physiol.  1852;   17  57. 
Rudolphi,  K.  A.     1807.     Anatomie  der  Pflanzen. 
Schacht,    H.     1850      Entwicklungsgeschichte   des   Pflanzenembryon.     Amsterdam 

1850.     In  Ann.  Sci.  Nat.  Bot.  Ill  15:  80-109.     1851. 
1858.     Ueber  Pflanzenbefruchtung.     Jahrb.  Wiss.  Bot.  1:  193-232.     pis.  11    15. 
Schimper,  A.  F.  W.     1880-1881.     Untersuchungen  iiber  die  Entstehung  der  Starke- 

korner.     Bot.  Zeit.  38:  881;  39:  185,  201,  217.     pis.  13,  2. 

1883.  Ueber  die  Entwicklung  der  Chlorophyllkorner  und  Farbkorper.     Bot.  Zeit. 
41:  105,  121,  137,  153.     pi.  1. 

1885.     Untersuchungen    iiber  die   Chlorophyllkorper   und   dio   ihnen   homologen 

Gebilde. 
Schleiden,   M.  J.     1837.     Einige  Blick  auf  die  Entwicklungsgeschichte  des  vege- 

tabilische  Organismus  bei  den  Phanerogamen.     Wiegmann's  Archiv.  1:  289. 
1838.     Beitrage  zur  Phytogenesis.     Muller's  Archiv  f.  Anat.  u.  Phys.     (English 

transl.,  Sydenham  Society,  1847. J 
1842.     Grundziige     zur     Wissenschaftlichen     Botanik.     Zweite     Aufl.     (English 

transl.  by  Lankester,  1849.) 
1844.     Bemerkung  zur  Bildungsgeschichte  des  Veg.  Embryo.     Flora  27:  787-789 
Schneider,   A.     1873.     Untersuchungen  iiber  Platelminthen      Jahrb.  d.   Oberh' 

Gesell.  Natur-Heilkunde  14.     Giessen. 
Schmitz,  Fr.     1879.     Untersuchungen  iiber  die  Zellkerne  der  Thallophyten.     Ver- 

handl.  Naturhist.  Ver.  Preuss.  Rheinl.  u.  Westf.     p.  346. 
Schneider,  A.     1883.     Das  Ei  und  seine  Befruchtung.     Breslau. 
Schultze,  M.     1861.     Uber  Muskelkorperchen  und  das  was  man  einc  Zelle  zu  uenneu 

hat.     Arch.  Anat.  u.  Physiol.     1-27. 
Schwann,  Th.     1839.     Mikroskopische  Untersuchungen  iiber  die  Uebereinstiinmung 

in    der    Struktur  und   dem   Wachsthum   der   Thiere   und    Pflanzen.     (English 

transl.,   Sydenham  Soc,    1847).     Preliminary  statement  in  Froriep-    Nbtizen, 

No.  91:  103,  112.     1838. 
Schweigger-Seidel,  O.     1865.     Ueber  die  Samenkorperchen  und  ihre  Entwicklung. 

Arch.  Mikr.  Anat.  1:  309-335.     pi.  19. 
Spallanzani,  L.      1786.     Experiences  pour  servir  a  l'histoire  de  la  genera t  ion  des  ani- 

maux  et  des  plantes.      Geneva. 
Sprengel,   C.    K.     1793.     Das  neu   entdekte   Geheimniss  der   Xatur   in    Ban    und 

Befruchtung  der  Blumen.     Berlin. 
Sprengel,  K.     1802.     Anleitung  zur  Kenntniss  der  Gewachs* 
Straburger,  E.     1875.     Ueber  Zellbildung  und  Zelltheilung.     Jena. 
1877.     Ueber  Befruchtung  und  Zelltheilung.     Jenaische  Zeitschr.  11. 
1879.     Die  Angiospermen  und  die  Gymnospermen.     Jena. 

1884.  Neue   Untersuchungen   iiber  die   Befruchtungsvorgang   bei   den    Phanero- 
gamen, als  Grundlage  fur  eine  Theorie  der  Zeugung.     Jena. 

1888.     Ueber  Kern-  und  Zelltheilung  im   Pflanzenreich,   aebsl   eiii   Anhang  uber 
Befruchtung.     Hist.  Beitr.  1. 
Thuret,   G.     1854-1855.     Recherches  sur  la   fecondation  des    Fucees,   Buivies  des 

observations  sur  les  antheridies  des  algucs.     Ann.  Sci.  Nat.  Bot.  [V,  2  and  3. 
Trevirantjs,  L.  C.     1806.     Vom  inwendigen  Ban  der  Gewachse. 

1811.     Beitrage  zur  Pflanzenphysiologie. 
Tulasne,  L.  R.     1849.     Etudes  d'embryogenie  v6ge"tale.     Ann.  Sci.  Nat.  Bot.  III. 
12:  21-137.     pis.  3-7. 


22  INTRODUCTION  TO  CYTOLOGY 

Unger,  F.     1844.     Leber  das  Wachsthum  der  Internodien,  von  anatomischer  Seite 
betrachtet.     Bot.  Zeit.     IMeristematische  Zellbildung.     Ibid. 

Virchow,  II.     1855.     Cellular  Pathologie.     Arch.  Path.  Anat.  Physiol.  8. 
1858.     Die  ( Vllularpathologie,  usw.     (Transl.  by  Chance,   1860.) 

Weismann,    A.     1885.     Die    Kontinuitat    des    Keimplasmas    als    Grunglage    einer 
Theorie  der  Vererbung.     Jena. 

Whitman,    C.    0.     1S78.     The   embryology   of   Clepsine.     Quar.    Jour.    Micr.    Sci. 
18:    215-315.     pis.  12-15. 

Wolff,  C.  F      1759.     Theoria  Generations. 


CHAPTER  II 

PRELIMINARY  DESCRIPTION  OF  THE  CELL 

In  a  survey  of  the  evolution  of  biological  science  it  is  noticeable  that, 
while  diverging;  lines  of  inquiry  have  broadened  the  field  of  view,  the 
attention  of  investigators,  speaking  generally,  has  been  directed  in  turn 
to  successively  smaller  constituent  parts  of  the  organism.  For  many 
years  plants  and  animals  were  studied  chiefly  as  whole-.  But  very 
early  there  were  made  many  scattered  observations  on  the  various  or- 
gans and  tissues  composing  the  body,  and  from  these  relatively  crude 
beginnings  morphology  and  histology  later  arose.  Again,  when  the 
protoplasmic  mass  which  we  know  as  the  cell  came  to  be  recognized  as 
the  unit  of  structure  and  of  function,  it  was  evident  thai  the  problems 
it  presents  should  be  investigated  to  a  certain  extent  by  themselves,  and 
such  investigation  is  the  task  of  modern  cytology. 

Within  the  field  of  cytology  itself  the  focus  of  attention  has  gradu- 
ally shortened.  While  many  workers  occupied  themselves  with  a  study 
of  the  general  behavior  of  the  cell  nucleus,  others  devoted  their  efforts 
entirely  to  an  investigation  of  its  important  constituent  elements,  the 
chromosomes.  Furthermore,  cytologists  at  present  are  much  interested 
in  knowing  whether  or  not  any  smaller  units,  corresponding  to  the 
"genes"  of  the  geneticist,  can  be  directly  demonstrated,  and  whether 
or  not  the  chromatic  granules  or  "chromomeres"  are  of  significance  in 
this  respect. 

In  the  course  of  all  such  studies  there  are  encountered  questions 
which  must  be  referred  ultimately  to  the  chemical  molecules  and  atoms 
and  their  interactions  within  the  cell,  so  that  biochemistry  may  in  a 
measure  be  looked  upon  as  a  department  of  cytology,  just  as  it  is  to  be 
regarded  in  other  respects  as  a  subdivision  of  chemistry.  The  subject  of 
cytology  thus  occupies  an  important  position  in  the  system  of  natural 
sciences.  It  stands  with  chemistry  and  physics  on  the  one  hand  and  the 
complex  phenomena  peculiar  to  living  organisms  on  the  other;  and  the 
steady  mutual  approach  of  the  physico-chemical  and  biological  fields 
is  due  in  large  measure  to  the  results  of  morphological  and  physiological 
studies  on  the  cell. 

For  the  term  cell  we  are  indebted  to  Robert  Eooke  and  the  othei 
microscopists  of  the  seventeenth  century,  who  applied  it  to  the  small 
cavities  in  the  honeycomb-like  structure  which  they  discovered  in  plant 
tissues.  Today  the  term  denotes  primarily  the  protoplasmic  'cell 
contents,"  which,  straneelv  enough,  the  early  worlcers  regarded  as  an 


24 


INTRODUCTION  TO  CYTOLOGY 


unimportant  fluid  product.  The  term  protoplast,  proposed  by  Hanstein 
(1880),  is  more  appropriate  and  is  coming  into  more  general  use,  but  long- 
usage  and  brevity  have  probably  insured  the  permanence  of  the  older 
term. 


Fig.  1. — Diagram  of  the  cell,  showing  its  principal  constituent  parts. 
A,  centrosphere,  with  centrosome  and  aster.     B,  nucleolus  or  plasmosome.     C,  nuclear 
membrane.     D,    nucleus,    filled   with   karyolymph.     E,    nuclear   reticulum,    composed    of 
linin    and    chromatin.     F,    plastid.     G,    metaplasmic    inclusion.     H,    chondriosomes.     /, 
vacuole.     J,  tonoplast  or  vacuolar  membrane.     K,  cytoplasm.     L,  ectoplast. 

Description  of  the  Cell. — The  morphology  of  the  cell  will  here  be 
sketched  only  in  its  barest  outlines,  by  way  of  introduction  to  the  detailed 
descriptions  presented  in  the  subsequent  chapters. 

The  two  most  constant  components  of  the  cell  (Fig.  1)  are  the 
cytoplasm,    in    which    the    other    cell    organs    are    imbedded,    and    the 


nOKRTT  LIBRARY 


n 


PRELIM IS ARY   DESCRIPTION  OP   THE  (ELL 


25 


nucleus,  which  at  least  in  many  respects  is  the  most   important  of  th< 
organs.1 

The  cytoplasm,  a  more  or  less  transparent,  viscous,  granular  Quid, 
may,  with  its  inclusions,  occupy  the  whole  volume  of  the  cell.  This  is 
generally  true  of  animal  cells  and  the  younger  cells  of  plants.  If  the 
cell  is  vacuolate,  as  is  usually  the  case  in  the  mature  plant  cell,  the 
cytoplasm  may  constitute  only  a  thin  layer  lining  the  wall,  the  central 
vacuole  with  its  cell  sap  often  far  exceeding  it   in  volume   (Fig.  2,  C). 


Fig.  2. 

A-C,  diagram  of  a  plant  cell  in  three  stages  of  development:  the  vacuoles  increase  in 
volume  and  the  protoplasm  becomes  limited  to  the  parietal  region.  D,  cell  of  stamen  hail 
of  Tradescantia,  showing  streaming  movements  in  the  cytoplasmic  strands.  K.  paren- 
chyma cell  from  cortex  of  Polygonella,  showing  nucleus,  plastids,  and  scanty  cytoplasm. 


In  many  cases  the  cytoplasm  forms  a  system  of  anastomosing  strands  thai 
often  show  active  streaming  (Fig.  2,  D).  Externally  the  cytoplasm  is 
limited  by  a  layer  of  different  consistency,  the  ylasma  membrane,  or 
ectoplast.  Where  it  comes  in  contact  with  the  enclosed  vacuole  it  is  also 
limited  by  a  membrane,  the  vacuole  membrane,  or  tonoplast. 

The  nucleus  is  bounded  by  a  nuclear  membrane  and  contains  an 
extremely  clear  fluid,  the  nuclear  sap,  or  karyolymph.  In  the  karyo- 
lymph  is  imbedded  the  nuclear  reticulum,  composed  usually  of  hnin,  an 
achromatic  supporting  material,  and  chromatin,  the  'nuclear  substance 
par  excellence."  One  or  more  true  nucleoli,  or  plasmosomes,  are  commonly 
present  in  the  nucleus,  and  may  or  may  not  be  closely  associated  with 
the  reticulum.  There  are  often  present  also  chromatin  nucleoli,  orkaryo- 
somes,  which  represent  accumulations  of  chromatin  at  certain  point-  oil 
the  reticulum,  and  should  not  be  confused  with  the  true  nucleoli. 

According  to  the  older  usage  the  extra-nuclear  portion  of  the  protoplast  was 
called  "protoplasm,"  which  was  unfortunate  because  we  now  know  that  the  nucleus 
also  is  composed  of  protoplasm,  or  living  substance  in  its  broader  Bense.  It  is  now 
the  general  custom  to  avoid  this  ambiguity  by  employing  Strasburger's  terms  cytoplasm 
and  nucleoplasm  (karyoplasm,  Flemming).  The  older  usage,  however,  has  not  been 
entirely  superseded. 


26  INTRODUCTION  TO  CYTOLOGY 

There  arc  usually  plastids  of  one  or  more  kinds  in  the  cytoplasm, 
the  most  conspicuous  in  plant  cells  being  the  green  chloroplasts. 

A  centrosomt  is  presenl  in  the  majority  of  animal  cells  and  in  those  of 
certain  lower  plants.  It  may  occupy  the  center  of  a  visibly  differentiated 
region,  the  centrosphere  or  attraction  sphere,  and  at  the  time  of  cell- 
division  is  the  focus  of  a  conspicuous  system  of  radiating  astral  rays, 
collectively  known  as  the  aster. 

Ckondriosomes  have  now  been  demonstrated  in  the  cells  of  nearly  all 
plant  and  animal  groups.  These  are  minute  bodies  having  the  form  of 
granules,  rods,  or  threads,  and  apparently  constitute  a  group  of  materials 
having  various  functions. 

Metaplasmic  inclusions  arc  accumulations  of  food  materials  and 
differentiation  products  that  are  relatively  passive.  These  non-proto- 
plasmic bodies  may  exist  in  the  form  of  droplets  or  crystals,  and  those 
which  are  not  transitory  or  reserve  food  materials  apparently  play  a  very 
minor  role  in  the  life  of  the  cell. 

Strictly  speaking,  the  cell  wall  as  at  present  understood  is  not  a  part 
of  the  cell  proper,  or  protoplast,  but  is  rather  regarded  by  many  as  a 
secretion  of  the  latter.  In  many  cells,  particularly  those  of  animals  and 
the  motile  cells  of  alga?  and  flagellates,  it  may  be  absent. 

The  foregoing  is  a  bare  sketch  of  the  general  structure  of  a  "typical" 
cell.  It  is  scarcely  necessary  to  point  out  that  the  cell  should  not  be 
thought  of  as  a  static  thing  with  a  permanent  physical  structure:  it  is 
rather  a  dynamic  system  in  a  constantly  changing  state  of  molecular 
flux,  its  constitution  at  any  given  moment  being  dependent  upon  ante- 
cedent states  and  upon  environmental  conditions.  As  stated  by  Moore 
(1912),  'the  living  cell  may  be  regarded,  from  the  physico-chemical 
point  of  view,  as  a  peculiar  energy  transformer,  through  which  a  continu- 
ally varying  flux  of  energy  ceaselessly  goes  on,  and  the  whole  life  of  the 
cell  is  an  expression  of  variations  and  alterations  in  rates  of  flow  of 
energy,  and  of  swings  in  the  balance  between  various  forms  of  energy." 
In  the  words  of  Harper  (1919),  the  cell  is  a  colloidal  system  in  which  the 
various  processes  have  become  progressively  localized  in  certain  regions, 
with  the  resulting  formation  of  organs,  which,  with  the  increasing  con- 
stancy of  the  processes  involved,  have  come  to  possess  a  permanence 
and  individuality  of  their  own.  In  view  of  the  spatial  relationship  and 
definite  physiological  integration  of  the  various  components  of  the  cell, 
we  are  to  look  upon  the  cell  not  as  a  mere  mixture  of  complex  substances, 
but  as  a  definitely  organized  system. 

The  Differentiation  of  Cells.— It  is  a  striking  fact  that  in  spite  of 
many  minor  variations  the  fundamental  structure  of  the  cell  is  essentially 
similar  in  nearly  all  living  organisms,  and  in  all  the  kinds  of  tissues 
which  go  to  make  up  the  body  of  any  one  of  them.  As  Harper  (1919) 
remarks,  '" evolution  as  we  know  it  has  not  consisted  in  the  production 


PRELIMINARY  DESCRIPTIOX  OF  THE  (ELL 


27 


of  now  typos  of  protoplasmic  structure  or  cellular  organization,  bul  in 
the  development  of  constantly  greater  specialization  and  division  of 
labor  between  larger  and  larger  groups  of  colls."     ( me  obvious  reason  for 

the  fundamental  similarity  of  the  cells  of  widely  different  tissues  is  found 
in  the  fact  that  all  of  them  are  derived  by  progressive  modification  from 
relatively  undifferentiated  "embryonic"  or  "  meristematic' !l  cells  during 
the  course  of  the  ontogeny.  In  a  young  vascular  plant,  for  example, 
definite  regions  (meristems)  consisting  of  such  colls  are  present  in  the 
root  tip  and  stem  tip,  and,  at  a  later  stage  of  development  in  many 
cases,  in  the  cambium  also.  As  a  general  rule  these  meristematic  cells 
are  without  large  vacuoles  or  other  conspicuous  products  of  differei  it  iation, 
and  are  separated  by  no  intercellular  spaces.  They  undergo  successive 
divisions  very  rapidly  (hence  the  use  of  root  tips  for  the  study  of  mil  osifl  ; 
and  while  some  of  the  products  of  division  become  greatly  modified  in 
structure  in  connection  with  their  specialization  in  function,  others 
retaintheir  embryonic  or  meristematic  character  and  continue  to  produce 
new  cells  from  which  new  tissues  are  built  up  throughout  the  life  of  the 
plant. 

In  the  bryophytes  and  pteridophytes  the  meristematic  activity  of  the 
apex  (root  tip  or  stem  tip,  or  apical  region  of  thallus)  usually  centers  in 
a  single  " apical  cell"  of  definite  shape,  which  cuts 
off  segments  (daughter  cells)  from  its  various  faces 
with  great  geometrical  regularity.  In  the  Mar- 
chantiales  and  Anthoceros  the  apical  cell  is  cuneate 
(wedge-shaped)  and  forms  segments  from  four  of 
its  faces;  in  the  anacrogynous  Jungermanniales 
it  is  sometimes  cuneate  but  more  often  dolabrate 
(ax-shaped)  and  produces  segments  from  its  two 
lateral  faces ;  and  in  the  acrogynous  Jungermanniales 
and  mosses  it  has  the  form  of  a  triangular  pyramid, 
cutting  off  segments  from  its  three  lateral  faces. 
This  last  type  is  found  also  in  the  pteridophytes: 

in  the  stem  tip  it  produces  segments  from  its  three    section  of  the  root  tip  of 
i    ,        i   c  i  •       ,,  i.  A-       •      „,m.-f;^,,     Osmund, i,    Bhowing    1 1  *  *  - 

lateral  faces,  whereas  in  the  root  tip,  in  addition    triangular       pyramidal 

to  these  three  series  of  segments,  it  cuts  off  from  its    apical  cell,     x  144. 

distally  directed  face  a  fourth  series,  which  becomes 

the  tissue  of  the  root  cap  (Fig.  3).     In  the  higher  vascular  plant  -  there 

is  no  single  cell  characteristically  different  from  the  others  of  the  apical 

meristematic  group. 

Most  of  the  visible  characters  which  ordinarily  serve  to  distinguish 
the  various  kinds  of  differentiated  cells  of  the  vascular  plant  are  found 
in  the  cell  wall  rather  than  in  the  protoplast  itself;  strictly  speaking, 
such  characters  are  histological  rather  than  cytological.  Thus  we  have, 
besides  meristematic  and  little  modified  parenchymatous  ells  (Fig.  2, 


28 


JSTRODVCTION   TO  CYTOLOGY 


E),  a  number  of  other  types,  such  as  tracheids,  vessels,  wood  fibers, 
sclerenchyma  fibers,  and  sieve  tubes  (Eig.  4),  all  of  which  are  characterized 
by  the  peculiar  \v;iys  in  which  their  walls  become  modified  through  sec- 
ondary and  tertiary  thickenings,  and  by  the  form  and  arrangement  as- 
sumed by  the  pits.  (See  p.  191.)  The  protoplasts  may  finally  disappear 
completely  from  wood  cells,  leaving  a  tissue  or  framework  composed  of 
lifeless  cell  walls. 


© 


© 


® 


® 


Fig.  4. — Differentiated  cells  from  vascular  plants. 

A,  wood  fiber  with  thickened  wall.  B,  C,  portions  of  tracheids  with  spiral  and  annular 
thickenings.  D,  pitted  tracheid.  E,  portion  of  sieve  tube  with  adjacent  companion  cells. 
F,  face  view  of  sieve  plate  shown  in  section  in  E. 


All  of  this  variety  of  form  and  structure  is  conditioned  by  varied 
functional  activity  on  the  part  of  different  protoplasts:  in  the  process  of 
cell  differentiation  morphological  and  physiological  changes  stand  in  the 
closest  relationship.  All  functional  differences  are  accompanied  by 
chemical  or  physical  differences  of  some  sort  in  the  protoplasm,  but  it 
is  mainly  in  the  non-protoplasmic  inclusions  and  secretions  (including 
the  wall)  rather  than  in  any  conspicuous  structural  changes  in  the 
protoplast  itself  that  cell  differentiation  is  rendered  visible  in  the  case 
of  plants.  Apart  from  differences  in  shape,  amount  of  vacuolar  material, 
accumulated  food,  and  other  products  of  differentiation  (see  p.  133), 
protoplasts  performing  widely  different  functions  may  appear  much  alike. 

Structural  differentiation  in  connection  with  division  of  labor  is  very 
striking  in  animal  cells,  which  are  destitute  of  such  walls  as  plant  cells 
possess.  The  muscle  cell  shows  many  fine  longitudinal  fibrillse  which 
have  to  do  with  the  cell's  power  of  contractility.  In  certain  muscles 
these  fibrillar  are  so  segmented  that  the  whole  cell,  or  muscle  fiber,  has  a 
transversely  striped  appearance   (Fig.   5,  F).     The  nerve  cell   (Fig.  5, 


PRELIMINARY  DESCRIPTION  OF   THE  CELL 


29 


i  tin  iiii 

..^.  .•••' ...  • 

llllllfll 

•  •  *  •  t  *  »  1 1  •  i  i  ■   < 

iliHJNMJI 

;fiij«'jP'jjj 


Fig.  5. — Nerve  and  muscle  cells  of  animals. 
A,  diagram  of  a  typical  neuron:  a,  axis  cylinder  process  or  axon,  ending  in  arborescent 
system;  d,  dendrites.  (After  Obersteiner  and  Hill.)  B,  cell  from  human  spinal  rord, 
X  75.  (After  Obersteiner  and  Hill.)  C,  nerve  cell  from  the  eye.  (After  L<  ithossek.)  D. 
Nerve  cell  from  the  earthworm.  (After  Kowalski.)  E,  young  voluntary  musrle  cell. 
F,  portion  of  mature  voluntary  muscle  cell,  showing  striations.  G,  Involuntary  muscle 
cell  from  intestine.      (E-G  after  Piersol.) 


c 


Fig.  »;. 

A,  connective  tissue  from  the  jelly  fish,  showing  branching  cells  and  elastic  fibers 
imbedded  in  gelatinous  matrix.  (After  Lang.)  H.  cells  of  hyaline  cartilage  imbedded 
in  their  secretion.     C,  blood  fell  from  chick  embryo. 


30 


INTRODUCTION   TO  CYTOLOGY 


A-D)  typically  possesses  a  single  unbranched  prolongation  (axon)  and 
one  or  more  others  (dendrites)  which  often  become  very  elaborately 
branched,  especially  in  the  ganglion  cells  of  the  spinal  cord  and  brain. 
The  cytoplasm  of  the  nerve  cell  contains  fine  fibrils,  and  also  granules 
of  chromatic  "Nissl  substance."  Cells  specialized  in  connection  with 
motility  such  as  spermatozoa  (Fig.  103)  and  the  cells  of  certain  epithelial 
tissues  (  Fig.  36),  show  complex  structural  modifications  not  only  in  the 
flagellse,  cilia,  and  cirri  which  they  bear  (p.  45),  but  also  in  the  other 
cell  organs  with  which  the  activities  of  these  motile  structures  are  closely 


I";.  7. — Paramoeeium    caudatum.     Semidiagrammatic    figure    showing    principal    parts. 

C.  V.,  contractile  vacuoles.  T,  trichocyst.  N,  n,  mega-  and  micronuclei.  P,  peri- 
atomial  groove.  M,  mouth.  0,  oesophagus,  with  undulating  membrane,  U.  M.  F.  V., 
food  vacuoles.      (After  Lang.) 


connected.  (See  Chapter  IV.)  Secretory  cells  are  often  distinguishable 
not  only  by  the  accumulations  of  secretion  products  in  their  cytoplasm, 
but  also  by  the  peculiar  forms  assumed  by  their  nuclei  (Fig.  17,  A,  C). 
The  cells  of  connective  tissue  (Fig.  6,  A)  form  many  long  interlacing  pro- 
cesses and  lie  in  a  supporting  matrix  which  represents  their  secretions. 
(  artilage  and  bone  cells  (Fig.  6,  B)  are  likewise  imbedded  in  their  secre- 
tions, which  are  here  produced  in  relatively  enormous  amounts  and,  where 
present,  constitute  the  main  supporting  framework  of  the  body. 

We  thus  see  that  the  life  of  the  complex  multicellular  organism  is 
dependent  upon  the  correlated  activities  of  a  multitude  of  cells  per- 
forming many  diverse  special  functions.     It  is  a  remarkable  fact  that 


PRELIMINARY  DESCRIPTION   OF   THE  <  ELL  3J 

all  of  the  functions  delegated,  as  it  were,  to  different  cells  (contractility, 
motility,  mechanical  support,  the  reception  and  conduction  of  stimuli. 
secretion,  and  excretion),  as  well  as  those  general  function-  common  to 
all  cells  (nutrition  and  reproduction  by  division),  may  in  the  protozoa 
and  protophyta  be  carried  on  within  the  limits  of  a  single  cell.  Such  a 
cell  as,  for  example,  the  body  of  a  Paramcechmi  (Fig.  7),  exhibits  a  cor- 
responding regional  differentiation  in  structure,  certain  functions  being 
localized  in  definitely  developed  organs.  Differentiation  is  therefore 
something  which,  fundamentally,  does  not  require  multicellular  -tinc- 
ture for  its  expression;  in  fact  the  most  important  single  step  ever  taken 
m  differentiation  was  that  which  set  apart  nucleus  and  cytoplasm,  giving 
the  type  of  organic  unit  common  to  all  subsequently  evolved  organisms. 
It  is  further  evident,  however,  that  the  evolution  of  the  higher  organisms 
has  unquestionably  been  very  largely  conditioned  by  the  multicellular 
state,  and  has  involved  a  progressive  division  of  labor  in  a  very  real  sent 
The  many  functions  of  a  single  cell  have  become  distributed  among  a 
number  of  cells  in  such  a  way  that  there  has  been  produced  a  harmonious 
whole  which  is  efficient,  adaptable,  and  progressive  to  a  degree  not  other- 
wise attainable. 

Bibliography  2 

See  Bibliography  \A  for  general  works  on  the  cell  and  for  reviews  of  early  cell 
literature.     For  the  latter  see  especially  the  works  of  Boveri,  Flemming,  Koernicke, 
Mark,    Meves,    Ruckert,    Waldeyer,    Whitman,    and    Zimniermann.     Other    works 
referred  to  in  Chapter  II: 
Hanstein,  J.     1880.     Das  Protoplasma  als  Trager  dcr  pflanzlichen  und  thierischen 

Lebensverrichtungen .     Heidelberg . 
Harper,  R.  A.     1919.     The  structure  of  protoplasm.     Am.  Jour.  Bot.  6:  273-300. 
Moore,  B.     1912.     The  Origin  and  Nature  of  Life.     X.  V.  and  London. 


CHAPTER  III 

PROTOPLASM 

In  his  famous  essay  on  protoplasm  in  1868  Huxley  very  fittingly 
referred  to  it  as  "the  physical  basis  of  life."  With  a  realization  of  the 
full  significance  of  this  phrase  there  comes  the  conviction  that  protoplasm 
is  the  most  interesting  and  important  substance  to  which  we  can  turn  our 
attention,  for  with  it  the  phenomena  of  life,  in  so  far  as  we  know  them, 
are  invariably  associated. 

In  spite  of  the  enormous  amount  of  work  which  has  been  done  upon 
protoplasm  during  many  years,  our  knowledge  of  it  must  still  be  regarded 
as  very  superficial  and  fragmentary.  We  can  scarcely  yet  say  definitely 
that  a  given  kind  of  protoplasm  is  not  a  single  complex  chemical  com- 
pound, as  is  held  by  one  prominent  school  of  biochemists:  all  ordinary 
analysis  seems  to  indicate  that  it  represents  a  somewhat  looser  combina- 
tion of  substances,  many  of  which  are  in  turn  very  elaborate  in  composi- 
tion; and  further  that  these  substances  probably  differ  from  those  found 
elsewhere  not  in  any  fundamental  manner,  but  only  in  the  degree  of  their 
complexity.  Proteins,  fats,  crystalloids,  water,  and  other  compounds 
make  up  protoplasm,  but  protoplasm  is  not  a  mere  mixture  of  these 
materials;  it  is  organized — it  is  a  system  of  complex  substances,  the 
activities  of  which  are  fully  coordinated.  Only  if  we  recognize  in  pro- 
toplasm an  organization  can  we  conceive  of  it  as  a  physico-chemical 
substratum  for  those  peculiar  orderly  activities  characterizing  living 
substance,  namely,  synthetic  metabolism,  reproduction,  irritability,  and 
adaptive  response. 

Physical    Properties. — Certain    early    ideas    regarding    the    physical 
nature  of  protoplasm  may  be  briefly  reviewed  at  this  point. 

Protoplasm  appeared  to  its  earliest  observers  merely  as  a  colorless, 
viscid  substance  containing  minute  granules.  Two  general  opinions 
soon  developed :  some  held  that  protoplasm  consists  of  but  a  single  fluid, 
whereas  others  regarded  it  as  a,  combination  of  two  fluids.  Brucke 
(1861),  who  was  one  of  the  first  to  lay  emphasis  on  the  fact  that  pro- 
toplasm is  an  organized  substance,  looked  upon  the  cell  body  as  a  con- 
tractile, semi-solid  material  through  which  there  streams  a  fluid  carrying 
granules.  Similar  to  this  was  the  idea  of  Cienkowski  (1863),  who  be- 
lieved he  saw  in  the  protoplasm  of  myxomycetes  two  fluids,  one  of  them 
hyaline  and  only  semi-fluid  (the  "ground  substance"),  and  the  other 
a  more  limpid  fluid  with  granules  suspended  in  it.     DejBary  (1859, 

32 


PROTOPLASM  :;;; 

1864),  on  the  other  hand,  regarded  protoplasm  as  a  single  semi-fluid  sub- 
stance, contractile  throughout,  but  showing  many  local  differences  due  to 
varying  water  content.  To  this  general  view  the  work  of  Hanstein 
(1870,  1880,  1882)  lent  support. 

Much  more  prominent  have  been  the  structural  theories  associated 
with  the  names  of  Klein,  Flemming,  Altnian,  and  Butschli,  and  known 
respectively  as  the  " reticular,"  "fibrillar/1  "granular,"  and  "alveolar' 
t  heories. 

The  reticular  theory,  which  was  formulated  by  Fromman  (1865, 
1876,  1884),  was  developed  especially  by  Klein  (1878-9)  and  supported 
by  van  Beneden,  Carnoy,  Leydig,  and  others.  These  workers  saw  in 
protoplasm  a  reticulum  or  fine  network  of  a  rather  solid  substance 
(spongioplasm),  which  holds  a  fluid  and  granules  in  its  meshes.  This 
view  was  adopted  for  a  time  by  Strasburger. 

The  fibrillar,  or  filar,  theory,  announced  by  Velten  (1873-6)  as  a 
result  of  his  observations  on  Tradescantia  and  other  forms,  stated  that 
protoplasm  is  composed  of  fine  fibrils,  which,  though  often  branched, 
do  not  form  a  continuous  network.  This  idea  was  developed  mainly  by 
Flemming  (1882),  who  called  the  substance  of  the  fibrils  mitome  and  the 
fluid  bathing  them  paramitome.  Some  observers  asserted  that  the  fibrils 
are  in  reality  minute  canals  filled  with  a  liquid,  the  granules  seen  by 
others  being  merely  sections  of  these  canals.  An  extreme  view  was  thai 
of  Schneider  (1891),  who  thought  the  entire  cell  might  consist  of  but  a 
single  greatly  convoluted  filament. 

To  the  followers  of  the  reticular  and  fibrillar  theories  the  fluid  held 
between  the  fibers  was  known  variously  as  ground  substance,  enchyU  ma 
(Hanstein  1880),  hyaloplasma  (Hanstein),  paramitome,  and  inter-Jltar 
substance.     The  granules  were  known  generally  as  microsomes  \  Hanstein). 

According  to  the  granular  theory  protoplasm  is  a  compound  of  in- 
numerable minute  granules  which  alone  form  the  essential  active  basis  for 
the  phenomena  exhibited;  the  observed  fibrillar  and  alveolar  structures 
are  of  secondary  importance.  Martin  (1881)  held  that  fibrils  and  net- 
works are  due  entirely  to  certain  arrangements  of  these  granules,  <>r 
microsomes.  Pfitzner  (1883)  pointed  out  that  the  granules  are  semi- 
solid and  float  in  a  more  fluid  ground  substance.  For  Alt  man  I  L886,et< 
who  was  the  most  prominent  exponent  of  the  theory,  the  granules  were 
actual  elementary  living  units,  or  bioplasts,  the  Liquid  containing  them 
being  a  non-living  hyaloplasm.  The  cell  was  therefore  looked  upon  not 
as  a  unit,  but  as  an  assemblage1  of  bioplasts,  "like  bacteria  in  a  zodglcea," 
and  the  bioplasts  were  believed  to  arise  only  by  division  of  others  <>t  their 
kind  (omne  gran  alum  e  granulo!). 

The  alveolar  theory,  also  known  as  the  emulsion,  or  foam,  theory,  was 
elaborated  principally  by  Butschli  (1S82,  etc.),  and  is  of  special  interest 
in  view  of  our  present-day  notions  of  protoplasmic  structure.     According 


34  INTRODUCTION  TO  CYTOLOGY 

to  Butschli  protoplasm  consists  of  minute  droplets  (averaging  1/*  in 
diameter)  of  a  liquid  "  alveolar  substance ';  (enchylema)  suspended  in 
another  continuous  liquid  "interalveolar  substance."  The  structure 
is  therefore  that  of  an  extremely  fine  emulsion,  and  the  appearances 
described  by  other  workers  are  due  to  optical  effects  encountered  in 
examining  the  minute  alveolar  structure.  Butschli  supported  his  theory 
by  making  artificial  emulsions  with  soaps  and  oils  which  showed  amoeboid 
movement  and  other  striking  resemblances  to  living  protoplasm. 

The  above  four  theories  have  been  termed  "monomorphic  theories," 
for  the  reason  that  each  of  them  stated  that  protoplasm  has  a  single 
characteristic  physical  structure.  Strasburger  in  1892  and  thereafter 
maintained  that  the  protoplast  is  regularly  composed  of  two  portions; 
an  active  fibrillar  kinoplasm,  concerned  primarily  with  the  motor  work  of 
the  cell,  and  a  less  active  alveolar  trophoplas?n,  chiefly  nutritive  in  func- 
tion. It  was  shown  by  von  Kolliker,  Unna  (1895),  and  others,  moreover, 
that  one  type  of  structure  may  be  transformed  into  another.  Flemming 
later  adopted  the  view  that  no  single  type  characterizes  protoplasm,  but 
that  the  latter  may  be  homogeneous,  alveolar,  fibrillar,  or  granular — 
i.e.,  it  is  " polymorphic."  Wilson  (1899)  found  that  all  four  states  are 
successively  passed  through  in  the  echinoderm  egg.  This  observation, 
which  was  made  upon  both  living  and  fixed  material,  showed  in  a  striking 
manner  the  colloidal  nature  of  protoplasm  (see  below),  since  it  is  now 
known  that  colloids  may  assume  very  diverse  structures  under  the  in- 
fluence of  changing  environmental  conditions.  The  work  of  A.  Fischer 
(1899),  who  treated  non-living  proteins  with  cytological  fixing  reagents 
and  so  produced  artifacts  similar  to  alveolae,  reticula,  and  granules,  should 
make  one  cautious  in  drawing  conclusions  regarding  protoplasmic 
structure  from  fixed  material.  It  should  be  understood  that  the  only 
trustworthy  observations  are  those  which  are  made  at  least  in  part  on 
living  material,  for  it  is  not  difficult  to  discover  all  four  types  of  struc- 
ture in  prepared  slides:  the  protoplasm  has  there  been  coagulated  by 
fixing  reagents,  and  we  know  that  in  the  coagulation  of  such  substances 
as  compose  protoplasm  an  entirely  new  structure  may  be  assumed. 

Protoplasm  as  a  Colloidal  System. — For  adequate  reasons  it  is  now 
customary  to  speak  of  protoplasm  in  terms  of  the  physics  and  chemistry 
of  colloids.  Colloids  are  those  glue-like  substances  which  are  uncrystal- 
line,  semi-solid,  and  very  slightly  or  not  at  all  osmotic.  They  have  a 
high  surface  tension,  coagulate  readily,  and  conduct  the  electric  current 
very  poorly.  They  are  "  disperse  heterogeneous  sytems,  i.e.,  they  consist 
essentially  of  particles  larger  than  molecules  of  a  substance  or  substances 
in  a  medium  of  dispersion  which  may  be  water  or  some  other  fluid '; 
(Child  19151).     The  particles  range  in  size  from  those  visible  to  the  naked 

1  These  paragraphs  on  colloids  are  based  largely  upon  the  convenient  summary 
given  by  Child  (1915,  pp.  20  ff.).  See  also  Czapek  (19116),  Baylies  (1915),  Hatschek 
(1916),  Bechhold  (1919),  and  Robertson  (1920). 


PROTOPLASM  •  35 

eye  down  to  single  molecules  and  ions.  In  the  latter  case  we  have  a  hue 
solution:  between  colloid  and  crystalloid  the  line  of  demarcation  is  thus  a 
purely  arbitrary  one.  In  many  cases  the  suspended  particles  are  too 
small  to  be  seen  with  the  ordinary  microscope,  which  will  not  rendei 
visible  a  body  with  a  diameter  less  than  about  0.20  to  0.25  ju\  but  with 
the  ultramicroscope,  which  will  reveal  particles  about  one-fortieth  of 
this  size,  they  may  be  clearly  seen.  Again,  the  ultramicroscope  is  insuffi- 
cient in  the  case  of  certain  colloids,  in  which  the  presence  of  suspended 
particles  can  still  be  shown,  however,  by  the  Tyndall  effect  (a  milky 
appearance  when  a  beam  of  light  is  passed  through  them).  Most  proto- 
plasmic colloids  are  of  this  last  type. 

In  a  colloidal  solution  the  particles  are  separate  from  one  another, 
(sol),  whereas  in  the  denser  "set"  condition  (gel)  they  are  more  closely 
aggregated  and  hence  not  free  to  move  upon  one  another.  A  colloid 
may  be  made  to  pass  from  the  sol  to  the  gel  state  or  vice  versa;  in  Borne 
cases  this  change  is  reversible,  but  in  others  it  is  not. 

Colloids  are  usually  classified  as  suspensoids  and  emulsoids.  Sus- 
pensoids,  in  which  the  particles  are  solid,  are  comparatively  unstable; 
are  readily  precipitated  or  coagulated  by  salts;  carry  a  constant  electric 
charge  of  definite  sign;  are  not  viscous;  do  not  show  a  lower  surface 
tension  than  that  of  the  medium  of  dispersion  alone;  and  are  mostly 
only  slightly  reversible.  Emulsoids,  in  which  the  suspended  particles 
are  fluid,  are  comparatively  stable;  are  less  readily  coagulated  by  salts; 
are  either  positively  or  negatively  charged;  are  usually  viscous;  have  a 
lower  surface  tension  than  the  medium  of  dispersion;  form  surface  mem- 
branes; and  are  highly  reversible.  Most  organic  colloids  are  emulsoids. 
and  there  can  be  no  doubt  that  many  of  the  characteristics  of  living  or- 
ganisms are  due  to  their  presence. 

In  an  emulsion  each  physically  homogenous  constituent  is  known  as 
a  phase.  In  mayonnaise  dressing,  to  cite  a  familiar  example,  there  are 
three  phases:  a  water  phase,  consisting  of  water  and  substances  dissolved 
in  it;  an  oil  phase;  and  a  protein  phase  (egg).  These  three  physically 
diverse  substances  are  brought  into  the  emulsified  state  by  beating:  one 
of  them  is  the  medium  of  dispersion  (external  phase)  and  the  others 
(internal  phases)  are  suspended  in  it  as  liquid  particles  or  droplets.  In 
such  an  emulsion  a  given  phase  usually  consists  of  more  than  one  chemi- 
cal substance:  the  water  phase,  for  example,  is  not  pure  water,  but  an 
aqueous  solution  of  salts  and  other  water-soluble  substances.  These  dif- 
ferent chemical  substances,  including  the  solvent,  which  make  up  a  single 
phase,  are  known  as  components. 

It  is  shown  by  certain  investigators  (Bancroft,  Clowes)  that  the  drop- 
lets of  a  suspended  phase  in  a  stable  emulsion  are  bounded  by  films  of 
different  constitution:  between  the  phases  of  an  alkaline  water-oil  emul- 
sion, for  example,  there  appear  to  be  delicate  films  of  a  soapy  nature. 


36 


ISTRODUCTION  TO  CYTOLOdY 


These  films  not  only  prevent  the  coalescence  of  the  droplets,  but  also, 
through  alterations  in  surface  tension,  influence  the  transposition  or 
inversion  of  phases  which  occurs  under  certain  conditions,  whereby  the 
suspended  phase  becomes  the  medium  of  dispersion  and  vice  versa  (Fig. 
8).  Such  inversion  probably  plays  an  important  role  in  many  cases  of 
1 1  a  information  of  sol  into  gel  and  of  gel  into  sol. 


Fig.  8. — Diagram  of  a  colloidal  emulsion,  illustrating  transformation  of  emulsion  of  oil 

in  water  to  emulsion  of  water  in  oil. 


A,  aqueous  phase.     B,  oil  or  other  non-aqueous  phase, 
dispersing  agent.      {After  Cloives,  1916.) 


C,  surface  film  of  soap  or  other 


The  properties  and  behavior  of  colloidal  substances  in  general  appear 
to  be  due  primarily  to  the  enormous  extent  of  the  reacting  surface  be- 
tween the  constituent  phases  which  results  from  the  finely  divided  state 
of  one  or  more  of  them.  In  the  accompanying  table  is  shown  the  amount 
of  surface  which  a  given  mass  of  matter  may  expose  when  subdivided 
into  successively  smaller  particles. 

Table.  Showing  the  Increase  of  Surface  with  the  Subdivision  of  1  c.c.  of 
Matter  in  the  Form  of  a  Cube  (Data  Partly  from  Hatschek,   1919.) 


Length  of  edge  of  cube 

Number  of  cubes 

Total  surface  exposed 

1  cm. 

1 

6  sq.  cm. 

1  mm. 

1,000 

60  sq.  cm. 

1()0M 

10fi 

600  sq.  cm. 

10u 

109 

6,000  sq.  cm. 

1m 

1012 

6  sq.  m. 

100/x/x 

1015 

60  sq.  m. 

1(W 

1018 

600  sq.  m. 

w 

1021 

6,000  sq.  m. 

The  evidence  at  hand  supports  the  view  that  protoplasm  is  essentially 
a  colloidal  solution  of  the  emulsion  type.  It  consists  of  at  least  three 
principal  phases:  a  water  phase,  containing  a  number  of  dissolved  compo- 


PROTOPLASM  37 

nents;  a  fat  phase,  consisting  of  fats  and  fat-soluble  components;  and  a 
complex  protein  phase.  Since  the  water  phase  is  here  the  medium  of 
dispersion  protoplasm  is  classed  as  a  "hydrosol'  in  its  ordinary  state, 
or  as  a  "hydrogel"  in  the  set  condition.  There  arc  doubtless  additional 
minor  phases  present,  protoplasm  being  in  reality  a  "complex  polyphase 
colloidal  system." 

The  presence  of  water  in  protoplasm  is  a  matter  of  fundamental  im- 
portance. As  emphatically  stated  by  Henderson  (1913),  ".  .  .  the 
physiologist  has  found  that  water  is  invariably  the  principal  constitu- 
ent of  active  living  organisms.  Water  is  ingested  in  greater  amounts 
than  all  other  substances  combined,  and  it  is  no  less  the  chief  excretion. 
It  is  the  vehicle  of  the  principal  foods  and  excretion  products,  for  most 
of  these  are  dissolved  as  they  enter  or  leave  the  body  [across  the  wall  of 
the  intestine  and  across  the  epithelia  of  kidneys,  lungs,  and  sweat  glands]. 
Indeed,  as  clearer  ideas  of  the  physico-chemical  organization  of  proto- 
plasm have  developed  it  has  become  evident  that  the  organism  itself  is 
essentially  an  aqueous  solution  in  which  are  spread  out  colloidal  sub- 
stances of  great  complexity  [Bechhold  1912].  As  a  result  of  these  condi- 
tions there  is  hardly  a  physiological  process  in  which  water  is  not  of 
fundamental  importance"  (pp.  75-77). 

The  amount  of  water  in  protoplasm  varies  greatly  under  different 
conditions,  but  normally  it  is  present  in  large  proportions.  It  makes  up 
85  to  95  per  cent  of  the  weight  of  actively  streaming  protoplasm  such 
is  seen  in  Elodea  and  Tradescantia,  and  in  actively  functioning  cells  it 
rarely  drops  below  70  per  cent.  In  dry  spores,  however,  it  may  be 
reduced  to  10  or  15  per  cent,  in  which  case  the  protoplasm  becomes  very 
viscous.  The  percentage  differs  constantly  in  different  parts  of  the  cell; 
nucleus,  cytoplasm,  and  plastids,  though  all  are  composed  primarily  of 
protoplasm,  contain  very  different -amounts  of  water.  Since  active  pro- 
toplasm is  a  liquid,  the  phenomena  of  surface  tension  and  other  propert  ies 
of  liquids  must  enter  largely  into  explanations  of  its  behavior. 

The  colloidal  nature  of  protoplasm  is  manifested  in  many  of  its  prop- 
erties. Its  power  of  adsorption,  which  lies  at  the  basis  of  many  cell 
reactions  and  certain  staining  processes,  is  similar  to  thai  of  other 
colloids.  Protoplasm,  like  other  colloids,  is  semi-permeable:  a  semi-per- 
meable region  is  probably  present  wherever  protoplasm  comes  in  contact 
with  other  substances,  such  as  water;  and  the  permeability  of  a  vacuolate 
cell  is  in  general  the  resultant  of  the  permeabilities  of  the  ectoplast, 
cytoplasm,  and  tonoplast. 

Protoplasm  shows  most  strikingly  its  colloidal  character  in  t  In-  changes 
of  physical  state  which  it  undergoes  as  the  effects  of  variations  in  the 
external  conditions.  The  alterations  due  to  changes  in  temperature  will 
serve  for  illustration.  Above  a  certain  temperature  the  colloid  gelatin 
exists  in  the  sol  state      it  is  a  hydrosol.     If  the  temperature  is  sufficiently 


38  INTRODUCTION  TO  CYTOLOGY 

lowered  the  gelatin  " sets"— it  becomes  a  hydrogel.  This  setting  is  a 
reversible  process:  if  the  temperature  is  again  raised  the  sol  state  is 
resumed.  On  the  contrary,  egg  albumen  is  a  hydrosol  at  ordinary 
temperatures  and  becomes  a  hydrogel  when  heated;  in  this  case  the 
change  is  an  actual  coagulation  and  is  not  reversible.  Many  of  the 
colloids  of  the  cell  are  of  this  non-reversible  type.  "The  evidence  that 
in  this  colloidal  condition  the  transition  from  liquid  to  solid,  from 
sol  to  gel,  tends  especially  to  pass  into  an  indefinite  series  of  gradations 
gave  a  basis  for  the  explanation  of  that  mixture  of  the  properties  of  solids 
and  liquids  which  has  puzzled  students  of  protoplasm"  (Harper  1919). 
Protoplasm  is  thus  easily  coagulated,  not  only  by  a  high  temperature, 
but  by  a  variety  of  chemical  substances.  The  "fixation'1  of  the  cell 
structures  by  the  reagents  employed  in  cytological  technique  is  primarily 
a  coagulation  phenomenon,  and  in  the  act  of  coagulation  a  substance, 
especially  one  as  complex  as  protoplasm,  undergoes  an  alteration  in 
physical  structure.  Although  such  fixing  fluids  preserve  very  well  the 
general  structure  of  the  cell,  the  effects  of  coagulation  should  always  be 
borne  in  mind  in  interpreting  finer  details  in  preparations  of  fixed  cells, 
and  in  evaluating  the  results  of  those  who  have  made  special  studies  on 
the  ultimate  structure  of  protoplasm. 

Microdissection.— Much  has  been  added  to  our  knowledge  of  the 
physical  nature  of  protoplasm  in  recent  years  through  microdissection. 
Certain  workers,  notably  Barber  (1911,  1914),  Kite  (1913),  Chambers 
(1914.  1915,  1917,  1918),  and  Seifriz  (1918,  1920)  have  developed  a 
technique  (fully  described  by  Barber  1914,  and  Chambers  1918)  whereby 
they  have  been  able  to  dissect  living  cells  under  the  high  powers  of  the 
microscope,  thus  opening  a  most  promising  field  for  investigation.  Kite, 
working  on  the  cells  of  several  plants  and  animals,  found  that  protoplasm 
exists  in  the  form  of  sols  and  gels  of  varying  consistency,  that  of  plant 
cells  being  as  a  rule  more  dilute  and  less  rigid  than  that  of  animals.  The 
cytoplasm  is  commonly  somewhat  more  viscous  than  is  usually  thought, 
having  the  consistence  of  a  "soft  gel,"  while  the  nucleus  may  often  be 
surprisingly  firm.     (See  Chapter  IV.) 

Chambers  (1917),  who  gives  a  convenient  bibliography  of  the  subject, 
states  that  in  the  early  germ  cells  and  eggs  of  certain  animals  the  proto- 
plasm is  in  the  sol  state  with  a  surface  layer  in  the  gel  state,  whereas 
adult  cells  are  usually  gels.  He  further  asserts  that  the  surface  gel  is 
readily  regenerated  after  injury,  a  new  gel  film  being  formed  over  the 
injured  area.  As  regards  the  structure  of  protoplasm,  he  finds  it  to 
consist  of  a  hyaline  fluid  carrying  microsomes  and  macrosomes,  which 
measure  less  than  1//.  and  from  2-4^  in  diameter  respectively.  Upon  dis- 
organization the  macrosomes,  which  are  more  sensitive  to  injury  than 
are  the  microsomes,  swell  and  go  into  solution,  while  the  hyaline  fluid 
flows  out  and  mixes  with  water  or  coagulates,  forming  a  reticular  or 
granular  structure. 


PROTOPLASM  39 

Seifriz  (1920)  has  investigated  with  care  the  viscosity  of  the  proto- 
plasms of  a  number  of  myxomycetes,  algae,  pollen  tubes,  protozoans, 
and  echinoderm  eggs.     He  finds  the  degree  of  viscosity  to  vary  widely, 

from  a  very  watery  to  a  fairly  rigid  gel  condition,  not  only  in  the  different 
organs  of  the  cell  but  also  in  the  protoplast  as  a  whole  at  different  Btag 
of  its  development.     He  warns  against  accepting  viscosity  alone  as  an 
index  of  the  gel  or  sol  state  of  the  protoplasm,  since  it  is  physical  structure 
and  not  viscosity  which  determines  these  states  in  an  emulsion. 

It  is  to  be  hoped  that  the  methods  employed  by  the  above  investi- 
gators will  be  further  developed  and  applied  more  widely,  for  through 
them  many  misconceptions  will  undoubtedly  be  collected. 

It  should  be  evident  from  all  these  considerations  thai  there  is  prob- 
ably no  sinlge  visible  structure  characteristic  of  protoplasm  at  all  times. 
Any  fundamental  structure  which  it  may  have  remains  to  be  discovered 
in  the  ultramicroscopic  constitution  of  the  colloids  and  other  materials 
of  which  it  is  composed,  and  in  the  physical  relations  which  these  beat 
to  one  another.  It  should  be  pointed  out,  however,  that  in  the  idea  of 
protoplasm  as  a  complex  colloidal  emulsion  we  have  the  best  hypothesis 
yet  offered  as  a  basis  for  the  interpretation  of  the  behavior  of  living- 
substance. 

Chemical  Nature  of  Protoplasm. — Chemically,  as  well  as  physically, 
protoplasm  is  exceedingly  complex,  and  the  study  of  its  constitution  has 
opened  a  field  of  research  which  is  continually  broadening.  ( )nly  a  brief 
summary  of  some  of  the  more  important  chemical  facts  can  be  presented 
here;  for  more  detailed  accounts  special  works  on  the  subject  must  be 
consulted.1 

As  already  pointed  out,  the  substances  of  which  protoplasm  is  com- 
posed are  probably  not  fundamentally  different  from  those  found  else- 
where,  but  show  rather  a  greater  complexity  and  a  high  degree  of 
organization.  Protoplasm  is  an  intricately  organized  system  of  water, 
proteins,  enzymes,  fatty  substances,  carbohydrates,  salts,  and  other 
minor  constituents.  The  often  cited  analysis  by  Reinke  and  Rodewald 
(1881)  of  the  myxomycete  sEthalium  septicum  (Fuligo)  showed  the  proto- 
plasm of  this  form  to  have  the  following  composition: 

PER  CENT  DRY  WEIGHT  PER  CENT  DB1    WEIGHT 

Proteins 40  ( Jholesterin    (lipoid  I 2.0 

Albumins  and  enzymes 15  Ca  salts  (except  CaCO  '»  5 

Other  N  compounds 2  Other  salts  d  5 

Carbohydrates 1-  Resins 12 

Fats 12  Undetermined   . .                 . .  6  5 

The  protein  matter  of  protoplasm  exists  in  relative  complex  forms. 
"The  chief  mass  of  the  protein  substances  of  the  cells  does  not  consisl  <»t' 

1  See  especially  the  hooks  of  Hammareten  (1909),  Wells  (1914),  Czapek  (1915  . 
Bayliss  (1915).  Mathews  (1916),  Palladia  (1918),  Robertson    L920  ,  and  thereviewby 

Zaeharias  (1909). 


40 


INTRODUCTION  TO  CYTOLOGY 


proteids  in  the  ordinary  sense,  but  consists  of  more  complex  phosphorized 
bodies  .  .  .'  (Hammarsten).  Such  "phosphorized  bodies"  are  the 
nucleo-proteins,  which  are  "probably  the  most  important  constituents 
of  the  cell,  both  in  quantity  and  in  relation  to  cell  activity"  (Wells). 
A  long  series  of  chemical  investigations  beginning  with  the  pioneer  work 
of  Miescher,  Hoppe-Seyler,  and  Reinke,  have  shown  that  these  nucleo- 
proteins  are  essentially  combinations  of  nucleic  acid  with  proteins,  or 
sometimes  with  the  simpler  histones  or  the  still  simpler  protamines. 

The  nucleus  as  a  rule  is  free  from  or  very  poor  in  uncombined  carbohy- 
drates, fats,  and  salts,  but  is  characterized  rather  by  the  abundance  of  a 
nucleo-protein  called  nuclein,  isolated  in  1871  by  Miescher,  who  gave  it  the 
formula  C29H49N9P3O22.  It  was  shown  by  Altman  (1889)  that  nuclein, 
like  the  other  nucleo-proteins,  could  be  split  into  two  substances:  nucleic 
acid  and  a  form  of  albumin  (protein),  the  two  existing  in  chemical  com- 
bination like  an  ordinary  salt.  Nucleic  acid  from  yeast  was  given  the  gen- 
eral formula  C10H59N14O22  —  2P205,  and  that  from  fish  sperm  C40H56N11O 

2P205.     Nucleic  acid  was  further  analysed  into  phosphoric  acid  and 


16 


certain  bases.  The  relation  of  these  simple  substances  to  nuclein,  and 
also  the  relation  of  nuclein  to  more  complex  nucleo-proteins,  are  shown  in 
the  following  scheme  (mainly  from  Wells): 


Higher 
nucleo- 
proteins 


Proteins 


Nuclein 


Proteins 

(albumins,  etc.) 

,  Nucleic  acid 


Phosphoric  acid 


Levulinic  acid 
Purin  bases 
Pyrimidins 
.  Pentoses 


Xanthin 
Guanin 
Adenin 
etc. 


The  nucleo-proteins  of  the  nucleus  (chromatin)  contain  very  little  of 
the  protein  constituent  and  are  thus  relatively  rich  in  phosphorus. 
( ilaser  (1916)  accordingly  speaks  of  chromatin  as  "a  conjugated  phospho- 
protein  group  with  a  nucleic  acid  group,  the  latter  group  being  a  complex 
of  phosphoric  acid  and  a  nuclein  base."  Kossel  (1889,  1891,  1893) 
even  concluded  that  in  certain  instances  (during  mitosis)  chromatin 
might  be  simply  nucleic  acid. 

In  the  cytoplasm,  on  the  contrary,  the  proportion  of  the  protein 
constituents  is  relatively  high.  The  cytoplasm  probably  has  no  true 
nuclein,  but  is  rich  in  nucleo-albumins,  albumins,  globulins,  and  pep- 
tones, which,  unlike  nuclein,  have  little  or  no  phosphorus.  As  a  result 
its  reaction  is  alkaline,  in  contrast  to  the  acidity  of  the  nucleus.  Accord- 
ing to  Hammarsten  (1909),  "the  globulins  and  albumins  are  to  be  con- 
sidered as  nutritive  materials  for  the  cell  or  as  destructive  products  in 


PROTOPLASM  11 

the  chemical  ( ransformat  ion  of  )  he  protoplasm. "     ( rranules  of  "  volut  in  ' 
formed  in  the  cytoplasm  are  also  looked  upon  as  :i  food  substance  used 
by  the  nucleus  in  the  elaboration  of  chromatin. 

The  fatty  components  of  the  cell  comprise  both  ordinary  fata  and 
lipoids  (fat-like  bodies  nol  decomposed  by  alkalis);  among  the  Latter 
lecithin  and  cholesterin  are  of  great  importance,  particularly  in  the  cells 
of  animals. 

The  carbohydrates  found  in  protoplasm  are  chiefly  pentoses  and 
hexoses,  which  are  as  a  rule  combined  with  proteins  and  with  lipoids. 
Glycogen  exists  free  in  the  cells  of  many  tissues  and  serves  as  a  source  of 
heat  and  energy.  The  important  role  played  by  pentosans  in  the  activity 
of  the  plant  cell  is  strongly  emphasized  by  Spoehr  (1919)  and  Macdougal 
(1920);  in  fact  these  authors  speak  of  protoplasm  as  "an  intermeshed 
pentosan-protein  colloid." 

Inorganic  salts  are  present  in  considerable  variety,  as  shown  by  the 
presence  of  the  following  elements  in  the  ash  of  Fuligo  protoplasm:  CI, 
S,  P,  K,  Mg,  Na,  Ca,  Fe. 

Because  of  their  failure  to  find  any  new  types  of  chemical  compound- 
in  their  analysis  of  protoplasm  Reinke  and  Rodewald  (1881)  thought  it 
probable  that  the  peculiarities  of  protoplasm  are  due  to  its  structure 
rather  than  to  its  chemical  composition.  It  has  since  been  found,  how- 
ever, that  certain  of  the  life  processes  continue  for  a  time  alter  the  pro- 
toplasm has  been  ground  up  mechanically.  Moreover,  more  refined 
analytical  methods  have  enabled  chemists  to  isolate  from  protoplasm 
certain  extremely  complex  and  unstable  proteins  (the  "protoplasmids,J 
of  Etard),  which  differ  greatly  in  degree  of  complexity,  if  not  otherwise 
from  proteins  encountered  elsewhere. 

Varieties  of  Protoplasm. — From  the  foregoing  resume  it  is  plain  that 
in  protoplasm,  because  of  the  many  combinations  possible  among  con- 
stituents present  in  such  great  variety,  we  have  a  substance  which  may 
exist  in  a  vast  number  of  different  forms.  When  it  is  further  recalled 
that  many  of  the  constituents  exhibit  singly  the  phenomenon  of  stereo- 
isomerism this  number  is  seen  to  be  incalculable.  For  example,  it  was 
shown  by  Miescher  that  an  albumin  molecule  with  to  carbon  atoms 
could  have  about  one  billion  stereoisomers,  and  some  albumins  probably 
have  more  than  700  carbon  atoms.  Albumin,  moreover,  is  only  one  of 
many  complex  substances  present  in  protoplasm.  Hence,  the  state- 
ment that  all  living  ('('lis  are  composed  of  the  same  substance,  proto- 
plasm, is  true  only  in  a  general  sense.  Although  they  are  made  up  of 
the  same  categories  of  substances  existing  in  the  same  general  type  of 
organization — the  hydrocolloidal  state  -the  protoplasms  of  different 
organisms  vary  widely  in  the  relative  amounts  of  these  leading  con- 
stituents. For  example,  the  lipoids  are  much  more  abundant  in  the 
protoplasm  of  animals  than  in  that   of  plants,   and  the  carbohydrate- 


42  INTRODUCTION  TO  CYTOLOGY 

protein  ratio  also  shows  notable  differences  in  the  two  kingdoms.  Ana- 
logous differences  also  exist  between  the  smaller  plant  and  animal  groups, 
and  with  these  differences  in  chemical  constitution  are  associated  many 
characteristic  diversities  in  metabolic  activity.  Thus  it  is  not  simply 
with  protoplasm  but  with  protoplasms  that  the  working  biologist  has 
to  deal. 

Special  emphasis  has  been  placed  upon  the  relation  of  this  great 
diversity  in  the  constitution  of  protoplasms  to  the  amazing  variety 
observed  among  living  organisms  by  Kossel,  Reichert,  and  a  number  of 
other  writers.  As  Reichert  states,  the  evidence  seems  to  indicate  that 
"in  different  organisms  corresponding  complex  organic  substances  that 
constitute  the  supreme  structural  components  of  protoplasm  and  the 
major  synthetic  products  of  protoplasmic  activity  are  not  in  any  case 
absolutely  identical  in  chemical  constitution,  and  that  each  substance 
may  exist  in  countless  modifications,  each  modification  being  character- 
istic of  the  form  of  protoplasm,  the  organ,  the  individual,  the  sex,  the 
species,  and  the  genus."  With  regard  to  the  integration  of  the  various 
protoplasmic  constituents,  Mathews  (1916)  says:  "Protoplasm,  that 
is  the  real  living  protoplast,  consists  of  a  gel,  or  sol,  which  is  composed 
of  the  colloids  of  an  unknown  nature  which  include  protein,  lipin  and 
carbohydrate.  Whether  these  colloidal  particles  consist  of  one  large 
colloidal  compound  in  which  enzymes,  protein,  phospholipin  and  car- 
bohydrate are  united  to  make  a  molecule  which  may  be  called  a  biogen 
[Verworn  1895,  1903],  cannot  be  definitely  stated,  but  it  seems  probable 
that  something  of  the  sort  is  the  case." 

The  Plasma  Membrane. — It  was  recognized  very  early  that  there  is 
at  the  surface  of  the  protoplast  a  thin  layer  of  relatively  resistant,  hya- 
line protoplasm  which  Hanstein  called  ectoplasm,  distinguishing  it  thus 
from  the  granular  endoplasm  within.  Pfeffer  (1890)  employed  the  cor- 
responding terms  hyaloplasm  and  polioplasm.  The  ectoplasmic  envelope, 
which  is  best  seen  on  "naked"  masses  of  protoplasm,  such  as  amoeba?, 
myxomycetes,  and  the  zoospores  and  gametes  of  algse,  has  been  variously 
referred  to  by  different  writers  as  the  ectoplast,  plasma  membrane,  Haul- 
schicht,  and  Plasmahaut.1 

The  proponents  of  the  reticular  and  fibrillar  theories  of  the  structure 
of  protoplasm  looked  upon  this  external  layer  as  a  region  in  which  the 
fibrils  are  more  closely  compacted  or  interwoven,  whereas  Butschli  re- 
garded its  relative  firmness  as  due  to  a  compact  radial  arrangement  of 
alveolae.  Pfeffer  (1890)  held  that  such  a  limiting  membrane,  which 
living  protoplasm  always  produces  on  an  exposed  surface  and  which 
consists  mostly  or  entirely  of  protein  substances,  is  not  itself  truly  proto- 
plasmic, whereas  the  majority  of  cytologists  have  thought  it  to  be  a 

1  A  discussion  of  ectoplasm  and  endoplasm  based  upon  a  large  number  of  ob- 
servations on  Amoeba  is  given  in  a  new  work  by  Schaeffer  (1920). 


PROTOPLASM  43 

special  protoplasmic  layer:  Strasburger,  for  instance,  believed  ii  to  be 
composed  of  kinoplasm. 

The  microdissection  studies  of  Kite  (1913)  and  Chambers  L917 
mentioned  above  have  extended  our  knowledge  of  the  physical  nature 
of  the  plasma  membrane.  Both  of  these  observers  describe  the  ecto- 
plast  of  an  Amceba  as  a  concentrated  gel.  Seifriz  (1918),  as  a  result  of 
such  studies  on  the  Fucus  egg  and  myxomycetes,  states  that  the  mem- 
brane is  a  definite  morphological  structure,  very  elastic  and  glutinous, 
and  capable  of  constant  repair.  He  further  asserts  that  membrane  form-" 
ation  is  a  physical  process  dependent  upon  the  physical  state  of  the 
protoplasm  and  not  upon  that  of  the  medium,  and  that  it  does  not  occur 
after  death. 

That  the  formation  of  such  a  limiting  membrane  at  the  surface  of 
protoplasm  is  the  result  of  the  tendency  of  colloidal  particles  to  accumu- 
late on  any  interface  has  been  pointed  out  by  physical  chemists.  ( 'it  ing, 
by  way  of  illustration,  the  film  which  forms  on  the  surface  of  cooling 
milk,  Moore  (1912)  says:  "The  chief  colloid  of  the  milk,  on  account  of 
its  affinities,  accumulates  on  the  surface,  the  accumulation  gives  increased 
concentration,  the  presence  of  the  increased  concentration  causes  the 
multi-molecules  to  build  together,  the  larger  molecules  fall  out  of  solu- 
tion as  particles,  and  these  join  to  form  a  close  network  or  film."  In  a 
similar  manner  the  unicellular  organism  or  other  mass  of  naked  proto- 
plasm develops  its  resistant  envelope,  and  the  enclosed  protoplast  of 
the  higher  plant  its  ectoplasmic  layer  and  tonoplast. 

Permeability. — The  physico-chemical  nature  of  the  plasma  mem- 
brane has  been  a  subject  of  much  discussion  among  physiologists.  <  hi 
the  assumption  that  the  permeability  of  the  cell  is  a  case  of  solubility  in 
the  ectoplasm,  E.  Overton  (1895,  1899,  1900)  developed  a  theory  of  the 
constitution  of  the  ectoplast.  It  was  pointed  out  first,  that  the  ectoplast 
is  not  miscible  with  water;  second,  that  in  plant  and  animal  cells  the  only 
bodies  which  are  not  miscible  with  water  in  the  ordinary  state  are  fats  and 
oils;  third,  that  the  ectoplast  is  more  or  less  permeable  to  substances  ac- 
cording as  the  latter  are  more  or  less  soluble  in  fats  and  oils;  and  fourth, 
that  any  substance  insoluble  in  another  substance  will  not  pass  through  a 
membrane  composed  of  the  latter.  It  was  therefore  concluded  that  the 
ectoplast  is  made  up  of  some  lipoid  compound,  such  as  lecithin,  which 
acts  as  a  semi-permeable  membrane.  This  theory,  though  very  sug- 
gestive, was  effectively  opposed  by  Ruhland  I  L909,  L915)  and  a  number 
of  other  investigators,  who  called  attention  to  many  substances  which 
do  not  behave  according  to  the  requirements  of  the  theory  stated  in 
so  simple  a  form.  A  more  nearly  adequate  conception  of  the  constitut  ion 
of  the  ectoplast  has  thus  been  souhgt. 

Of  the  more  recent  theories  which  have  been  offered  in  connection 
with  the  problem  of  permeability  the  most   promising  are  those  which 


44 


INTRODUCTION   TO  CYTOLOGY 


interpret  the  ectoplasl  as  an  emulsion.  According  to  Czapek  (1910, 
1911,  1915)  the  ectoplasl  is  an  emulsion  of  lipoids,  proteins,  and  other 

substances,  the  lipoids  forming  a  suspended  phase.  " Protoplasm  is  a 
colloidal  emulsion  of  lipoids  in  hydrocolloidal  media,  the  latter  containing 
proteins  and  mineral  salts."  Lepeschkin  (1910,  1911)  advanced  the 
contrary  view  that  the  lipoids  form  the  medium  of  dispersion.  In  at- 
tempting to  account  for  changes  in  permeability  Clowes  (1916)  points 
out  that  inversion  of  phases  probably  plays  an  important  role,  while 
Spaeth  (1916)  ascribes  changes  in  permeability  to  alterations  in  the 
degree  of  dispersion  of  the  colloids,  with  resulting  changes  in  the  vis- 
cosity  of  the  membrane.  A  more  definitely  stated  hypothesis  of  the 
latter  type  is  that  tentatively  suggested  by  Lloyd  (1915)  and  Free  (1918). 
Colloids  are  known  that  "have  two  liquid  phases  which  differ  in  composi- 
tion only  in  the  relative  proportion  of  water  and  of  the  substance  of  the 

colloid"  (Free).  It  is  accordingly  possible  that 
alterations  in  permeability  may  be  due  to  changes 
in  the  distribution  of  water  between  two  such 
phases  present  in  the  plasma  membrane.  When 
water  passes  from  the  internal  (suspended)  to  the 
external  (continuous)  phase,  the  droplets  of  the 
former  would  become  very  small;  when  the  move- 
ment is  in  the  opposite  direction  they  would  be- 
come very  large  and  closely  packed.  As  a  result 
there  would  be  such  changes  in  the  physical 
nature  of  the  membrane  as  would  aid  in  interpret - 
„  ,         ing  the  behavior  of  the  latter  toward  substances 

lie  9. — Amoeba,  snow- 
ing ectoplasm,  endo-  entering  or  leaving  the  cell.     It  is  held  that  such  a 

plasm    and   contractile  hypothesis  accounts  more  readily  for  the  gradual 

changes  in  permeability  observed  than  does  the 
inversion  theory  of  Clowes,  according  to  which  the  change  might  be  ex- 
pected to  occur  suddenly.  It  is  pointed  out,  however,  that  both 
processes  are  probably  involved. 

Whatever  the  degree  of  correspondence  between  the  above  inter- 
pretations and  reality  may  be,  it  is  scarcely  open  to  doubt,  especially 
since  the  work  of  Bancroft  (1913)  and  Clowes  (1916)  on  colloids,  that  in 
such  theories  we  have  our  best  prospect  of  reaching  an  adequate  knowl- 
edge of  the  plasma  membrane,. which,  because  of  its  great  importance  in 
the  life  of  the  cell,  is  to  be  regarded  as  a  definite  "osmotic  organ." 

Protozoa. — It  is  in  the  Protozoa  that  the  ectoplast  shows  its  most 
elaborate  structural  differentiations.  (See  Minchin,  1912,  Chapter  V.) 
Here  the  ectoplast  clearly  has  several  functions :  protective,  motor,  excre- 
tory, and  sensory.  In  most  forms  other  than  the  Sarcodina  there  is  a 
resistant  envelope  of  some  sort.  This  may  represent  (a)  the  entire  ecto- 
plast modified  (the  "periplast"  of  Flagellata);  (6)  a  superficial  modified 


ec  t.- 


PROTOPLASM 


i:> 


layer  of  the  ectopias!  (the  "pellicle"  of  Infusoria  and  some  Amoebae);  (c)  a 
secreted  layer  ("cell  membrane")  rather  than  a  modification  of  the 
ectoplast.     In  certain  cases  definite  actively  protective  organs,  the  tri- 

chocysts,  are  differentiated  in  the  ectoplasm. 

Among  the  ectoplasmic  structures  with  a  motor  function  the  simpl 
are  the  pseudopodia;  in  the  larger  ones  there  is  a  core  of  endoplasm     Fig. 
9),  but  the  more  delicate  "filose"  ones  consist  entirely  of  ectoplasm   I  Fig. 
10).     The  flagellum  of  Euglena  was  reported  by  Butschli  to  have  an  elas- 


v\\  I     f 


Fig.    11. 

A,  flagellum  of  Euglena,  showing  endoplasmic  core 
and  ectoplasmic  sheath.  {After  Butschli.)  B,  Trypano- 
soma  tincce,    with   undulating    membrane.     {After  Min- 

dopodia  composed  of  ectoplasm,     chin.)    C,  Trypanosoma  percoB,  showing  myonemes.       \Jttr 
{From  Minchin,  after  Schultze.)     Minchin.)    Z>,  flagellum  of  Euglena,      Vfier  Dellxnger.) 


Fig.   10. — Gromia   oviformis, 
showing   filose-reticulate   pseu- 


tic  endoplasmic  core  with  a  contractile  ectoplasmic  sheath  (Fig.  11.1 
but  the  later  figure  of  Dellinger  (1909)  represents  it  as  composed  of  tour 
twisted  filaments  ending  within  the  animal  as  a  system  of  branching 
rootlets  (Fig.  11,  D).  Cilia,  which  are  short  and  numerous  and  show 
rythmic  pulsation;  cirri,  which  are  formed  of  tufts  of  cilia;  membranellce, 
representing  fused  rows  of  cilia;  and  undulating  membranes,  which  are 
sheet-like  extensions  of  the  ectoplasm  (Fig.  11.  H).  arc  all  essentially 
ectoplasmic  organs.  A  further  motor  differentiation  is  Been  in  the 
minute  contractile  fibrils  known  as  myonemes,  which  are  analogous  to  a 


46 


IS 'PRODUCTION  TO  CYTOLOGY 


system  of  muscle  fibers  (Fig.  11,  C).  In  ciliated  forms  they  run  beneath 
the  rows  of  cilia. 

Contractile  vacuoles,  which  exercise  an  excretory  function,  originate 
in  the  ectoplasm,  although  later  they  may  lie  much  deeper. 

A  sensory  function  is  performed  by  the  "eyespot,"  which  is  sensitive 
to  light,  and  also  by  the  flagellar  and  cilia,  which  are  often  receptors  of 
tactile  stimuli.  The  eyespot  seems  in  some  instances  to  be  plastid-like 
in  character,  and  will  be  discussed  in  Chapter  VI. 

Protoplasmic  Connections.- -The  fine  protoplasmic  strands  (Plasmo- 
desmeri)  connecting  many  plant  cells  through  pores  in  the  intervening 
walls  are  extensions  of  the  ectoplasm  (Fig.  12).     Several  early  workers 


B  C 

Fig.   12. — Protoplasmic  connections  in  vascular  plants. 

A,  B,  Pinus  pinea:  cells  of  cotyledon.      X  375.      (After  Gardiner  and  Hill, 
Phytelephas    ("vegetable   ivory "):  endosperm    cells    with    greatly    thickened 
showing   spindle-shaped   bundles   of   connecting   strands,     m,    middle   lamella, 
grammatic. 


1901.)      C, 

walls     (w), 

Semidia- 


suspected  the  presence  of  such  connections  before  they  were  able  to  see 
them,  and  even  the  coarse  strands  passing  through  the  sieve  plates  of 
sieve  tubes,  though  often  observed,  were  not  well  known  until  the  time 
of  Hanstein's  work  in  1864.  The  finer  strands  of  other  plant  tissues 
where  described  in  a  large  number  of  researches  between  1880  and  1900. 
Among  these  may  be  mentioned  those  of  Wille  (1883)  and  Borzi  (1886)  on 
the  Cyanophycese;  Kohl  (1891),  Overton  (1889),  and  Meyer  (1896)  on 
the  Chlorophycese;  Hick  (1885)  on  the  Fucacese;  Hick  (1883),  Massee 
(1884),  and  Rosenvinge  (1888)  on  Floridese;  and,  on  vascular  plants, 
those  of  Tangl  (1879),  Russow  (1882),  Strasburger  (1882,  1901),  Goros- 
chankin  (1883),  Terletzki  (1884),  Wortman  (1887,  1889),  Haberlandt 
(1890),  Kienitz-Gerloff  (1891),  Jonsson  (1892),  Kuhla  (1900),  Gardiner 
(1884,  1897,  1900),  Hill  (1900,  1901),  and  Gardiner  and  Hill  (1901). 
i*  With  respect  to  the  origin  and  development  of  these  connecting 
strands  very  little  is  accurately  known.  Some  observers  have  claimed 
that  the  pores  through  which  they  pass  are  present  from  the  time  the 
primary  wall  is  first  formed,  no  wall  substance  being  laid  down  at  these 
points.     Gardiner  (1900)  believed  them  to  arise  directly  from  the  median 


PROTOPLASM  17 

portion  of  the  fibers  of  the  achromal  ic  figure  al  I  he  close  of  mitosis.  1 1  is 
observations  were  made  on  the  endosperm  of  Lilium  and  Tamus.  <  >i  hers, 
on  the  contrary,  have  regarded  them  as  secondarily  developed  si  rucl  nn 
Their  absence  from  the  walls  between  CuscvJta  and  Vixcum  and  their 
hosts  (Kienitz-Gerloff,  Kuhla,  Strasburger  1901),  and  also  from  many 
cells  which  glide  over  one  another  during  growth,  is  a  fact  opposed  to  the 
latter  interpretation.  Although  they  have  been  demonstrated  in  a 
number  of  kinds  of  tissue  they  probably  do  not  occur  so  widely  as  some 
have  supposed;  but  it  may  nevertheless  be  true  that  in  many  cases  their 
apparent  absence  is  due  to  the  fact  that  the  special  methods  often  neci 
sary  to  their  demonstration  have  not  been  widely  employed. 

As  to  their  function,  it  can  scarcely  be  doubted  that  they  may  serve 
to  transmit  stimuli  of  one  kind  or  another  from  cell  to  cell  (  Pfeflfer  1 896 
Noteworthy  in  this  connection  is  their  presence  in  tissues  of  plant  parts 
known  to  be  particularly  responsive  to  external  stimuli,  such  as  the  leaves 
of  Mimosa  (Gardiner  1884)  and  Dioncea  (Gardiner  1884;  Macfarlane 
1892),  the  stamens  of  Berberis  (Gardiner  1884),  and  the  sensitive  labellum 
of  the  orchid,  Masdevallia  muscosa  (Oliver  1888).  Their  extensive 
development  in  storage  tissues,  such  as  the  endosperm  of  seed-  (Tangl 
1879;  Gardiner  1897),  would  also  indicate  that  they  are  in  part  responsible 
for  the  readiness  with  which  nutritive  materials  are  translocated  insuch 
specialized  tissues. 

Vacuoles. — Vacuoles  in  the  cytoplasm  are  more  characteristic  of 
plant  than  of  animal  cells.  They  are  usually  absent  in  the  very  young 
cell,  but  appear  as  growth  and  differentiation  progress.  In  case  they  are 
very  small  and  numerous  the  cytoplasm  takes  on  an  alveolar  appearance. 
but  more  commonly  they  coalesce  to  form  one  lame  vacuole  which 
may  occupy  a  volume  greater  than  that  of  the  protoplast  itself  I  Fig.  2 
This  condition  is  characteristic  of  many  mature  cells  of  plants,  but  is 
comparatively  rare  in  animals. 

The  ordinary  vacuole  is  essentially  a  droplet  of  fluid,  consisting  ol 
water  with  differentiation  products  in  solution,  surrounded  by  a  delicate 
limiting  membrane.  DeVries  (1885)  developed  the  theory  thai  vacuoles 
are  derived  from  "tonoplasts."  The  tonoplasts  were  believed  to  be 
small  bodies  imbedded  in  the  cytoplasm  and  multiplying  by  fission. 
Through  the  absorption  of  water  they  swell  and  become  vacuoles,  the 
vacuole  wall  thus  being  made  up  of  the  material  of  the  tonoplasl  body. 
We  still  refer  to  the  vacuole  wall  as  the  tonoplast.  De  Vries  looked  upon 
the  vacuole  as  a  body  with  an  individuality  somewhat  similar  to  thai 
of  a  nucleus,  since  the  tonoplast  from  which  it  develops  was  supposed  to 
arise  from  a  preexisting  tonoplast  by  division.  The  theory  wassupported 
by  certain  other  workers,  but  it  does  not  enjoy  wide  acceptance  today 

It  has  been  found  by  Bensley  (1910)  and  others  thai  there  is  in  the 
cytoplasm  of  certain  comparatively  young  cells  a  system  of  fine  canals 


48 


INTRODUCTION  TO  CYTOLOGY 


-   l48t 


which  later  open  up  to  form  vacuoles  (Fig.  13).  The  fixing  reagents 
commonly  employed  in  cytological  technique  destroy  these  canalicalce; 
and  since  Bensley,  by  using  special  reagents,  demonstrated  such  canals 
in  the  familiar  cells  of  the  onion  root  tip,  it  is  highly  probable  that  they 
occur  very  widely.     It  seems  more  reasonable  oo  suppose  that  the  fluid 

differentiation  producos,  when  they  are  first 
forming,  gradually  come  to  move  along  certain 
paths,  forming  canals,  and  later  accumulate  in 
the  form  of  vacuoles,  than  to  suppose  that  the 
vacuoles  originate  in  such  individualized  units 
as  the  tonoplasts  of  deVries. 

Fluids  other  than  water  may  also  occur  in 
the  form  of  vacuoles;  oil  vacuoles,  for  example, 
are  not  uncommon  in  oertain  cells.  If  fats,  oils, 
and   other  products  of  metabolism   take   their 

Fig.  13.— Cell  from  root    origin  in  chondriosomes,  as  some  suppose  (see 
tip  of  Allium  cepa,  showing    Chapter  VI),  it  is  not  improbable  that  some- 

StoftO*     UftCr  Cham~    thing  at  least  anal°g°us  t0  the  above  mentioned 

tonoplast   behavior   may  occur  in   the  case  of 

certain   substances   appearing  in  the    cell.     The    cell    sap    and    other 

differentiation   products  in  the  cytoplasm  will  be  discussed  further  in 

Chapter  VII. 


PROTOPLASM  AS  THE  SUBSTRATUM  OF  LIFE 

Since  the  true  significance  of  protoplasm  was  first  recognized  in  the 
middle  of  the  last  century  many  suggestions  have  been  ventured  regard- 
ing the  nature  of  the  relation  existing  between  life  and  its  physical  basis. 
A  full  discussion  of  this  subject  obviously  cannot  be  entered  upon  here, 
but  theories  of  two  types,  the  micromeric  and  the  chemical,  may  be 
cited  by  way  of  illustration. 

Micromeric  Theories. — Many  years  ago  there  were  developed  certain 
speculative  " micromeric  theories"  of  the  constitution  of  protoplasm; 
these  became  particularly  prominent  during  the  latter  half  of  the  nine- 
teenth century.  According  to  these  " atomic  theories  of  biology"  the 
principle  of  life  was  held  to  reside  in  ultimate  fundamental  particles. 
The  particles  were  supposed  to  be  for  the  most  part  of  ultramicroscopic 
size,  capable  of  independent  growth  and  reproduction,  and  associated 
like  members  of  a  vast  colony  in  protoplasm.  Such  vital  units  were 
compared  by  some  to  chemical  molecules,  but  they  were  generally 
regarded  as  something  much  more  complex.  Examples  of  such  units 
were  the  "  organic  molecules"  of  Buff  on,  the  "microzymes"  of  Bechamp, 
the  " physiological  units"  of  Spencer,  the  " plastidules "  of  Maggi  and 
Haeckel.  the  "bioplasts"  of  Altman,  the  "vital  particles"  of  Wiesner, 


PROTOPLASM  19 

the  "gemmules,:i  of  Darwin,  the  "biophores"  of  Weismann,  the  "pan- 
gens"  of  de  Vries,  and  the  "ergatules"  and  "generatules"  of  Batschek. 

In  a  somewhat  similar  manner  a  number  of  the  Later  investigators 
occupied  with  the  study  of  the  ultimate  structure  of  protoplasm  have 
often  been  led  to  inquire  which  of  the  constituents  of  protoplasm  arc  the 
actually  living  elements.  Among  those  who  viewed  protoplasm  ae 
reticular  structure  some  held  the  material  of  the  reticulum  to  be  the  true 
living  substance,  the  liquid  ground  substance  being  lifeless,  wlierea- 
others  held  the  reverse  to  be  true.  Many  of  those  who  saw  in  protoplasm 
a  granular  structure  regarded  the  granules  as  the  ultimate  living  unit-, 
and  more  recently  there  has  even  been  a  tendency  on  the  part  of  some 
investigators  (Beijerinck,  Lepeschkin),  who  have  emphasized  the  emul- 
sion  nature  of  protoplasm,  to  view  the  droplets  of  the  suspended  phase  in 
a  similar  light.  To  Butschli  the  continuous  phase  was  the  essential 
substance. 

By  most  modern  biologists  such  attempts  to  assign  the  principle  of 
life  to  any  particular  constituent  unit  of  protoplasm  or  of  the  cell,  whether 
this  unit  be  an  observed  structural  component  or  a  purely  imaginary  one. 
are  regarded  as  not  in  harmony  with  an  adequate  modern  conception 
of  the  term  " living."  It  has  been  repeatedly  emphasized  that  life  should 
be  thought  of  not  as  a  property  of  any  particular  cell  constituent,  but 
as  an  attribute  of  the  cell  system  as  a  whole  (Wilson  1899);  or,  as  Brooks 
(1899)  put  it,  not  merely  as  a  property  but  as  a  relation  or  adjustment 
between  the  properties  of  the  organism  and  those  of  its  environment. 
This  recalls  Herbert  Spencer's  characterization  of  life  as  a  "continu- 
ous adjustment  of  internal  relations  to  external  relations."  As  Sachs 
(1892,  1895)  and  others  urged,  the  various  elements  in  the4  cell  should  be 
referred  to  as  active  and  passive  rather  than  living  and  lifeless.  These 
elements  play  various  roles  in  the  cell's  activity:  each  contributes  to  the 
orderly  operation  of  the  whole.  When  any  part  fails  to  function  properly, 
or  when  the  proper  adjustment  is  not  maintained,  the  whole  system  of 
correlated  reactions,  the  resultant  of  which  we  call  life,  must  become 
disorganized.  As  Child  (1915)  remarks,  the  theories  postulating  vital 
units  only  transfer  the  problems  of  life  from  the  organism  to  something 
smaller;  the  fundamental  problem  of  coordination  is  no  nearer  solution 
than  before,  and  the  whole  question  is  placed  outside  the  field  of  experi- 
mentation. Harper  (1919)  also  points  out  that  modern  cytology  DO 
longer  looks  upon  protoplasm  as  a  substance  with  a  single  specific  struc- 
ture, or  as  one  made  up  of  ultimate  fundamental  units  of  some  kind. 
but  rather  as  a  colloidal  system  or  group  of  systems  of  varying  structure 
and  composition.  "The  fundamental  organization  of  living  material  Is 
expressed  in  the  structure  of  the  cell."  The  cell  itself,  and  not  some 
hypothetical  corpuscle,  is  the  unit  of  organic  structure.  Protoplasm  is 
accordingly  not  made  up  of  structural  units  arranged  in  various  ways  to 
i 


50  INTRODUCTION  TO  CYTOLOGY 

form  the  cell  organs,  but  is  rather  a  colloidal  system  in  which  special 
processes  and  functions  have  become  localized  and  fixed  in  certain  regions; 
and  this  in  turn  has  resulted  in  the  evolution  of  organs  possessing  more 
or  less  permanence. 

Chemical  Theories. — Much  more  suggestive,  if  not  conclusive,  have 
been  certain  attempts  to  place  the  phenomena  of  the  organism  upon  a 
purely  chemical  basis.  With  the  development  of  organic  chemistry  from 
the  time  of  Wohler's  (1828)  synthesis  of  urea  onward  there  has  grown 
up  the  idea  that  life  processes  and  chemical  reaction  not  only  resemble 
each  other  but  are  actually  the  same  fundamentally.  When  protoplasm 
was  subjected  to  chemical  anatysis  and  found  to  consist  chiefly  of  water 
and  proteins,  and  when  these  substances  became  more  intimately  known, 
the  task  of  explaining  the  activity  of  protoplasm  in  terms  of  the  chemis- 
try of  proteins  was  undertaken.  One  group  of  workers  developed  the 
hypothesis  that  peculiarly  labile  protein  molecules  are  responsible  for 
the  organism's  reactions,  "death,;  being  primarily  a  change  from  the 
labile  to  the  stable  condition  on  the  part  of  these  molecules.  Such 
molecules  were  called  "biogens,;  by  Verworn  (1903).  The  molecule 
itself  was  not  thought  of  as  alive,  but  its  constitution  was  held  to  be  the 
basis  of  life,  which  "results  from  the  chemical  transformations  which  its 
lability  makes  possible."  Accordingly,  "life  itself  consists  in  chemical 
change,  not  in  chemical  constitution"  (Child  1915). 

A-dami  (1908,  1918)  contends  that  life  is  thus  "the  function,  or  sum  of 
functions,  of  a  special  order  of  molecules."  These  ultimate  molecules  of 
living  matter  he  calls  biophores  (not  to  be  confused  with  the  biophores 
of  Weismann,  which  were  molecular  complexes),  and  he  locates  them  in 
the  nucleus,  the  cytoplasm  having  merely  "subvital"  functions.  They 
are  proteidogenous  in  nature,  i.e.,  they  compose  an  active  substance 
which  takes  the  form  of  relatively  inert  proteins  when  subjected  to 
chemical  analysis.  The  biophore  is  conceived  by  Adami  to  have  the 
form  of  a  ring  or  a  ring  of  rings  of  the  benzene  type — a  ring  of  amino  acid 
radicles  with  many  unsatisfied  affinities  or  bonds.  The  biophore  grows 
in  a  manner  analogous  to  that  of  the  inorganic  crystal:  ions  and  radicles 
from  the  surrounding  medium  become  attached  as  side  chains  to  the  free 
bonds  of  the  central  ring  and  take  on  a  grouping  similar  to  that  of  the 
latter;  in  this  way  the  biophoric  molecules  are  multiplied.  Since  side 
chains  can  be  detached  and  new  ones  of  other  kinds  added,  the  biophore 
is  changeable  and  may  exist  in  many  different  forms.  Although  the 
central  ring  is  thought  to  be  relatively  stable  and  fixed,  the  variety  of  side 
chains  and  their  many  possible  arrangements  probably  give  to  each 
species  a  distinct  kind  of  biophore.  On  this  hypothesis  the  molecule  of 
living  matter  (biophore)  is  one  "of  extraordinary  complexity,  and  in  a 
state  of  constant  unsatisfaction,  built  up  by  linking  on  other  simple 
molecules,  and  as  constantly,  in  the  performance  of  function,  giving  up 


PROTOPLASM  :,l 

or  discharging  into  the  surrounding  medium  these  and  other  molecular 
complexes  which  it  has  elaborated"  (Adami  1918,  pp.  251  2  "  Ml 
vital  manifestations  are  manifestations  of  chemical  change  in  proteidogen- 
ous  matter,  are,  in  short,  the  outcome  of  arrangement  of  thai  matter 
with  the  necessary  liberation  or  storing  up  of  energy  "  |  p.  225  }.  Accord- 
ingly, life  is  "a  state  of  persistent  and  incomplete  recurrent  satisfaction 
and  dissatisfaction  of  .  .  .  certain  proteidogenous  molecules"  flQOS 
Vol.  I,  p.  r>5). 

Pictet  (1918)  also  associates  the  phenomena  of  life  with  a  Bpecial 
structure  of  the  organic  molecule.  Only  the  arrangement  of  t  he  a  toms  in 
open  chains,  he  asserts,  permits  the  manifestation  of  life  and  its  main- 
tenance; the  cyclic  structure  is  that  of  substances  which  have  losl  this 
faculty;  and  death  results,  from  the  chemical  point  of  view,  from  a 
cyclization  of  the  elements  of  the  protoplasm. 

To  the  theory  that  the  vital  processes  are  bound  up  with  a  special 
form   of   protein  or  protein-like  molecule  many  have   objected.     For 
example,  Hober  (1911)  has  contended  that  there  are  present   in  the 
organism    only   those  kinds  of  proteins  which  may  be  formed  in  the 
laboratory.     He  urges  that  life  should  not  be  thought  of  as  a   single 
process,  or  as  dependent  upon  any  particular  kind  of  molecule,  but  rather 
that  it  should  be  looked  upon  as  the  result  of  many  correlated  process 
occurring  between  many  substances  under  certain  conditions.     "If  we 
accept  this  idea,"  says  Child   (1915,  p.   19),   "we  must  abandon   the 
assumption  of  a  living  substance  in  the  sense  of  a  definite  chemical 
compound.     Life  is  a  complex  of  dynamic  processes  occurring  in  a  certain 
field  or  substratum.     Protoplasm,   instead   of   being  a  peculiar   living 
substance  with  a  peculiar  complex  morphological  structure  necessary  for 
life,  is  on  the  one  hand  a  colloidal  product  of  the  chemical  reactions,  and 
on  the  other  hand  a  substratum  in  which  the  reactions  occur  and  which 
influences  their  course  and  character  both  physically  and  chemically. 
In  short,  the  organism  is  a  physico-chemical  system  of  a  certain  kind.'' 
Harper  (1919)  is  also  opposed  to  theories  based  upon  the  conception 
of  protoplasm  as  a  single  complex  chemical  substance,  as  well  as  to  those 
which  hold  protoplasm  to  be  a  relatively  simple  two-phase  colloidal 
system— the  alveolar  and  granular  theories,  for  example.     "The  crude 
simplicity    and  general   inadequacy   of  these    .    .    .    concept  inn- 
have  done  much  to  bring  the  whole  subject  of  protoplasmic  organization 
into  disrepute.     On  the  other  hand  the  conception  of  protoplasm  as  an 
aggregate  of  complex  compounds,  a  polyphase  colloidal  system  or  system 
of  systems,  seems  to  do  much  more  adequate  justice  to  the   observed 
facts." 

Conclusion.  As  stated  at  the  opening  of  the  present  chapter  it  is 
with  protoplasm  that  the  phenomena  of  life,  in  so  far  a-  we  know  them, 
are  invariably  associated.     The  complex  behavior  of  the  living  organism 


52  INTRODUCTION  TO  CYTOLOGY 

can  receive  scientific  explanation  (i.e.,  be  fitted  into  an  orderly  scheme 
of  antecedents  and  consequents),  if  at  all,  only  on  the  basis  of  the 
constitution  and  properties  of  the  materials  composing  protoplasm;  the 
structural  organization  of  protoplasm;  the  relation  of  the  reactions  and 
n-sponses  of  protoplasm  in  the  form  of  organized  units  or  cells  to  the 
environmental  conditions;  the  chain  of  energy  changes  occurring  in 
connection  with  all  of  the  organism's  activities;  and  the  correlation  of 
all  these  conditions  and  events.  It  is  largely  the  effort  to  account  for 
organization  and  regulatory  correlation,  and  the  consequent  behavior  of 
the  complex  organism  as  a  versatile  and  consistent  unit  or  individual — as 
something  more  than  a  cell  aggregate — that  has  led  to  certain  present 
day  vitalistic  theories,  as  opposed  to  those  which  would  hold  life  to  be 
dependent  upon  "nothing  but':  the  correlated  physico-chemical  reac- 
tions and  interactions  occurring  in  protoplasm. 

Whatever  our  ultimate  judgment  in  this  matter  shall  be — for  any 
decision  at  present  is  premature — it  is  scarcely  to  be  denied  that  the 
hypotheses  that  have  thus  far  been  most  stimulating  to  research  in 
biological  science  and  most  valuable  in  analysing  the  data  afforded  by 
this  research  are  those  which  seek  to  formulate  vital  activity  in  terms  of 
what  the  physicist  for  convenience  calls  matter  and  energy;  and  which 
hold  life  to  be  not  the  manifestation  of  a  super-organic,  non-perceptual 
entity,  or  even  of  a  distinct  perceptual  but  hypothetical  vital  energy, 
but  rather  the  resultant  of  the  many  correlated  interactions  involving 
only  energies  of  known  kinds.  The  way  must  not  be  closed,  however, 
against  possible  new  categories  of  energy.  The  description  (reduction 
to  order)  of  our  perceptual  experience  of  organic  nature,  which  is  the 
primary  task  of  biological  science  and  which  has  been  scarcely  more  than 
begun,  must  for  the  present  be  made  as  far  as  possible  in  terms  applicable 
also  to  inorganic  nature.  It  is  here  that  achieved  results  would  seem  to 
justify  the  judicious  use  of  a  "mechanistic"  working  hypothesis,  whereby 
the  attempt  is  made  to  "describe  the  changes  in  organic  phenomena  by 
the  same  conceptual  shorthand  of  notation  as  suffices  to  describe  inor- 
ganic phenomena '!  (Pearson).  To  what  extent  our  ultimate  biological 
theory  is  to  show  the  need  of  non-mechanical  energies  or  principles  will 
depend  very  largely  upon  what  this  scientific  description  (orderly  formu- 
lation) turns  out  to  be  like  as  investigation  proceeds,  and  also  upon  the 
degree  of  success  with  which  the  physicist  will  resume  the  phenomena  of 
inorganic  nature  in  mechanical  formulae.  Thus,  as  Professor  D'Arcy  W. 
Thompson  forcefully  says : 

"While  we  keep  an  open  mind  on  this  question  of  vitalism,  or  while  we  lean, 
as  so  many  of  us  now  do,  or  even  cling  with  a  great  yearning,  to  the  belief  that 
something  other  than  the  physicalforces  animates  the  dust  of  which  we  are  made, 
it  is  rather  the  business  of  the  philosopher  than  of  the  biologist,  or  of  the  biologist 
only  when  he  has  served  his  humble  and  severe  apprenticeship  to  philosophy, 


PROTOPLASM  53 

to  deal  with  the  ultimate  problem.  It  is  the  plain  bounded  duty  of  the  bioloig 
to  pursue  his  course  unprejudiced  by  vitalistic  hypotheses,  along  the  road  of 
observation  and  experiment,  according  to  the  accepted  discipline  of  the  natural 
and  physical  sciences.  .  .  It  is  an  elementary  scientific  duty,  it  is  a  rule  thai 
Kant  himself  laid  down,  that  we  should  explain,  just  as  far  as  we  possibly  can, 
all  that  is  capable  of  such  explanation,  in  the  light  of  the  properi  tea  of  matter  and 
of  the  forms  of  energy  with  which  we  are  already  acquainted."  (Presidential 
address  before  the  Zoological  Sectionof  the  British  Association  for  the  Advance- 
ment of  Science,  1911.) 

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Miescher,    F.     1871.     Ueber    die    chemische    Zusammensetzung    der    Eiterzellen. 

Med.-Chem.  TJnters.  Lab.  ausgew.  Chem.  zu  Tubingen  4.     Berlin. 
1874.     Die     Spermatozoen     einiger     Wirbelthiere.     Verhandl.     d.     Naturforsch. 

Gesell.  in  Basel  6. 
Mixchin,  E.  A.     1912.     An  Introduction  to  the  Study  of  the  Protozoa.     London. 
Moore,  B.     1912.     The  Origin  and  Nature  of  Life.     New  York. 
Oliver,  F.  W.     1888.     On  the  sensitive  labellum  of  Masdevallia  muscosa  Rchb.  f. 

Ann.  Bot.  1:  237-253.     pi.  12. 
Overton,  E.     1889.     Ein  Beitrag  zur  Kenntniss  der  Gattung  Volvox.     Bot.   Centr. 

39:  65,  113,  145,  177,'  209,  241,  273.     pis.  1-4. 
1895.     Ueber  die  osmotischen  Eigenschaften  der  lebenden  Pflanzen-  und  Thierzelle. 

Vierteljahrsschr.  Nat.  Ges.  Zurich  40:  159-184. 

1899.  Ueber  die  allgemeinen  osmotischen  Eigenschaften  der  Zelle,  ihre  vermuth- 
lichen  Ursachen  und  ihre  Bedeutung  fur  die  Physiologic     Ibid.  44:  85-135. 

1900.  Studien   iiber   die   Aufnahme   der  Anilinfarben  durch  die  lebende     Zelle. 
Jahrb.  Wiss.  Bot.  34:  669-701.     See  Pfliiger's  Archiv  92. 

Palladix,  V.  I.     1918.     Plant  Physiology.     (Engl,  transl.,  ed.  by  Livingston.) 

Pearsox,  K.     1892.     The  Grammar  of  Science. 

Pictet,  A.     1919.     Life  and  the  structure  of  molecules.      (Geneva  Arch.  Phys.  et 

Xat.  Sci.)     Engl.  Trans,  in  Sci.  American  Suppl.  87 :  50-51,     59. 
Pfeffer,  W.     1885.     Zur  Kenntniss  der  Kontaktreize.     Unters.  Bot.  Inst.  Tubingen 
1:  483-535.     fig.  1. 
1890.     Zur  Kenntniss  der  Plasmahaut  und  der  Vacuolen,   nebst  Bemerkungen 
.iiber  den  Aggregatzustand  der  Protoplasmas  und  iiber  osmotische  Vorgange. 
Abh.  Math.-Phys.  kl.,  Sachs.  Ges.  Wiss.  16. 
1896.     Ueber  den  Einfluss  des  Zellkerns  auf  die  Bildung  der  Zellhaut. 
Pfitzxer,    W.     1883.     Beitrage    zur   Lehre   vom    Baue   des   Zellkerns   und   seinen 

Theilungserscheinungen.     Arch.  Mikr.  Anat.  22:  616-688.     pi.  25. 
Prixgsheim,  N.     1854.     Untersuchungen  iiber  Bau  und  Bildung  der  Pflanzenzelle. 
Berlin. 

Reichert,  E.  T.     1914.     The  germ  plasm  as  a  stereochemic  system.     Science  40: 
649-661. 


PROTOPLASM  57 

Reinke  und  Rodewald.     1881.     Die  chemische  Zusammensetzung  ilea  Protoplaan 

von  Mthalium  septicum.     Unters.   Bot.  Lab.   Gottingen. 
Reinke,    F.     1882.     Protoplasma-Probleme.     Ibid. 

1883.  Ein  Beitrag  zur  physiologischen  Chemie  von  Mthalium  septicum.  Ibid. 
Robertson,  T.  B.  1920.  Principles  of  Biochemistry.  Philadelphia  and  New  York. 
Rosenvinge,  L.  K.     1888.     Sur  la  formation  des  pores  secondairea  chez  lea  Polysi- 

phonia.     Bot.  Tidakr.  17:  10. 
Roskine,  G.     1917.     La  structure  des  myonemea.     Compt.  Rend.  Soc.  Biol.     Paria 

69:  363-364. 
Huhland,   W.     1909.     Beitrage  zur  Kenntniss  der  Permeabilita.1  der  Plasmahaut. 
Jahrb.  Wiss.  Bot,  46:  1-54.     figs.  2. 
1915.     Zur  Kritik  der  Lipoid-  und  Ultrafiltertheorie  der  Pla8mahaut,  uaw. 
Biochem.  Zeitschr.  54:  59-77. 
Russow.     1883.     Ueber  die  Perforation  der  Zellwand  und  den  Zusammenhang  der 
Protoplasmakorper  benachbarten  Zellen.     Sitzber.  Naturfor.  Gea.  Univ.  Dorpal 
6:5G2. 
von  Sachs,  J.  1892.     Physiologische  Notizen.     II.     Flora  75:  57-67. 

1895.     Physiologische  Notizen.     IX.  Weitere  Betrachtungen  iiber  Energiden  und 
Zellen.     Ibid.  81:  405-434.     (See  Wilson  1900,  p.  30.) 
Sch.effer,  A.  A.     1920.     Ameboid  Movement.     Princeton  Univ.  Press. 
Schneider,  C.     1891.     Untersuchungen  iiber  die  Zelle.     Arb.  Zool.  Inst.  Wien  9: 

179. 
Schwarz,  F.     1887.     Die  morphologische  und  physiologische  Zusammensetzung  des 

Protoplasmas.     (Rev.  in  Bot.  Zeit.  45  :  576-583.) 
Seifriz,  W.     1918.     Observations  on  the  structure  of  protoplasm  by  aid  of  microdis- 
section.    Biol.  Bull.  34 :  307-424.     figs.  3.     (Bibliography.) 
1920.     Viscosity  values  of  protoplasm  as  determined  by  microdissection.     Bot. 
Gaz.  70 :  360-386. 
Spaeth,  R.  A.     1916.     The  vital  equilibrium.     Science  43:  502-509. 
Spcehr,    H.    A.     1919.     The    carbohydrate    economy    of  cacti.     Carnegie   [nat.   of 

Wash.,  Publ.  287.  pp.  79. 
Strasburger,  E.     1882.     Ueber  den  Bau  und  das  Wachsthum  der  Zellhaute.     Jena. 
1892.     Schwarmsporen,  Gameten,  pflanzliche  Spermatozoiden  und  das  We8en  der 

Befruchtung.     Histol.  Beitr.  4. 
1901.     Ueber   Plasmaverbindungen   pflanzlicher    Zellen.     Jahrb.    Wiss.    Bot.    36: 
493-610.     pis.  14,  15. 
Tangl,  E.     1879.     Ueber  offene  Communicationen  zwiachen  den  Zellen  des  Endo- 
sperms einiger  Samen.     Ibid.  12:  170-190.     pis.  4-6. 
Terletzki,  P.     1884.     Anatomie  der  Vegetationsorgane  von  StruthiopU  ris  gi  rmanica 

und  Pteris  aquilina.     Jahrb.  Wiss.  Bot,  15:  452-501.     pis.  24   26. 
Unna,  P.     1895.     Ueber  die  neueren  Protoplaamatheorien,  und  daa  Spongioplasma. 

Deu.  Med.  Zeit.  1895.     p.  98. 
Velten,  W.     1876.    Die  phyaikaliache  Beachaffenheit  dea  pflanzlichen  Protoplaamas. 

Sitzber.  Akad.  Wiss.  Wien,  Math.-Nat.     Kl.,  73:  1   131    151. 
Verworn,  M.     1895.     Allgemeine  Phyaiologie.     Jena. 

1903.     Die  Biogenhypotheae.     Jena. 
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Wiss.  Bot.  16 :  465-598. 
Wells,  H.  G.     1914.     Chemical  Pathology.     2d  edition.     Philadelphia.     Chapter  I. 
W'ille,  N.     1883.     Ueber  die  Zellkerne  und  die  Poren  der  Wande  bei  den  Phyco- 

chromaceen.     Ber.  Deu.  Bot.  Ge8.  1:  243-246. 


:»s  INTRODUCTION   TO  CYTOLOGY 

Wilson,  E.  B.     L899.     On  protoplasmic  structure,  in  the  eggs  of  echinoderms  and 

some  other  animals.     Jour.  Morph.  15:  Suppl.  1-23. 
W  ortmann,   J.     1887.     Zur   Kenntniss  der  Reizbewegungen.     Bot    Zeit      45-785 

801,  817,  833. 

1889.     Ueber  die  Beziehungen  der  Reizbewegungen  wachsender  Organe  zu  den 
normalen  AVachsthumerscheinungen.     Ibid.  47:  453,  469,  485. 

Zachamas,  E.  1881-1893.  Ueber  die  chemische  Beschaffenheit  des  Zellkerns. 
Bot.  Zeit.  39:  169.  Ueber  den  Zellkern.  Ibid.  40:  611.  Ueber  Eiweiss,  Nuclein, 
imd  Flastin.  Ibid.  41:  209.  Ueber  den  Nukleolus.  Ibid.  43:  257.  Beitrage 
zur  Kenntniss  des  Zellkerns  and  der  Sexualzellen.  Ibid.  45 :  281.  Ueber  Chroma- 
tophilie.     Ber.  Deu.  Bot.  Ges.  11:  188-195. 

Zimmermann,    A.     1893.     Sammel-Referate   2,    3.     Beih.    Bot.  Centr    3:  209-217 
321-328. 


CHAPTER   EV 

THE  NUCLEUS 

It  is  now  half  a  century  since  the  modern  period  of  cytology  was 
ushered  in  by  a  series  of  researches  revealing  the  remarkable  behavior 
of  the  nucleus  during  the  critical  stages  of  the  life  cycle  Because  of 
the  peculiarly  intimate  relation  which  this  behavior  has  been  shown  to 
have  to  many  outstanding  biological  problems,  including  thai  of  heredity, 
it  is  largely  in  nuclear  phenomena  that  cytological  interesl  has  con- 
tinued to  center  throughout  the  period.  The  most  striking  of  th< 
phenomena  form  the  subjects  of  several  subsequent  chapters:  at  this 
point  we  shall  consider  the  nucleus  only  as  it  appears  in  the  "resting' 
cell,  i.e.,  in  the  cell  not  undergoing  division. 

Occurrence. — The  most  conspicuous,  and  in  some  respects  the  mosl 
important  of  the  cell  organs  is  the  nucleus.  Whether  or  not  we  shall  Bay 
that  every  living  cell  contains  a  nucleus  will  depend  upon  what  we  are  to 
include  under  the  term.  If  the  chromatin  or  chromatin-like  substances, 
no  matter  whether  distributed  throughout  the  cell  in  the  form  of  granules 
or  aggregated  to  form  a  well  defined  organ,  be  regarded  as  constituting  a 
nucleus,  then  it  follows  that  all  plant  and  animal  cells  normally  have 
nuclei.  If,  however,  as  certain  protozoologists  prefer,  the  term  nucleus 
be  employed  only  with  reference  to  a  distinctly  delimited  organ,  we  must 
regard  those  lowly  organized  cells  with  scattered  chromatic  material  as 
devoid  of  nuclei,  although  they  possess,  as  all  cells  apparently  do,  material 
which  performs  at  least  the  nutritive  functions  of  a  nucleus.  This  latter 
type  of  organization,  which  is  found  in  certain  members  of  the  Protozoa 
and  Bacteria,  and  also  some  Cyanophyceae,  will  be  discussed  later  on  in 
connection  with  nuclear  structure  (p.  66)  and  cell-division  (Chapter  X 
In  myxomycetes,  where  simple  and  primitive  conditions  mighl  be  expected, 
Jahn  (fl)08,  1911)  and  Olive  (1907)  have  demonstrated  the  presence  of 
definite  nuclei  showing  mitosis  and  the  phenomenon  of  chromosome 
reduction. 

General  Characters. --The  vast  majority  of  cells  have  one  nucleus 
each.  A  few  exceptions  may  be  noted.  In  tapetal  cells,  laticiferous  ves- 
sels, the  internodal  cells  of  Characeae,  and  certain  other  cells  there  are 
often  several  nuclei  arising  by  the  division  of  one.  In  the  Siphone®  among 
green  algse  (Fig.  14,  V)  and  the  Phycomycetes  among  fungi  there  arc  no 
cross  walls  in  the  filamentous  and  much  branched  vegetative  body,  SO  that 

59 


60 


IXTRODUCTION  TO  CYTOLOGY 


large  numbers  of  nuclei  are  associated  in  one  extensive  mass  of  cyto- 
plasm. Such  a  body  is  called  a  ccenocyte,  and  the  coenocytic  condition  is 
found  in  a  number  of  the  lower  organisms.  In  the  Uredinese  (rusts)  the 
typical  life  history  is  made  up  of  two  phases,  with  uninucleate  and 
binucleate  cells  respectively.  In  certain  Infusoria  two  kinds  of  nuclei 
are  regularly  present.  Thus  in  Par amcecium  caudatum  (Fig.  15)  there  is 
one  small  micronucleus  which  divides  by  a  peculiar  form  of  mitosis,  and 
one  large  meganucleus  (macronucleus)  which  divides  amitotically.  In 
P.  aurelia  there  are  two  micronuclei  and  one  meganucleus,  whereas  in 


Fig.   14. 

V,  portion  of  body  of  Vaucheria,  showing 
coenocytic  condition;  nuclei  dark  and 
plastids  in  outline.  C,  portion  of  body  of 
Cladophora,  showing  semi-coenocytic 
condition. 


Fig.  15. — Par  amcecium  caudatum  un- 
dergoing fission;  mega-  and  micronuclei 
dividing.  {From  Minchin,  after  Biitsehli 
and  Schewiakoff.) 


Stentor  there  may  be  one  meganucleus  and  several  micronuclei.  In 
general  the  meganucleus  seems  to  be  a  storage  organ  of  the  cell;  it  may 
disappear  and  be  replaced  by  a  new  one.  The  micronucleus  performs 
the  usual  nuclear  functions.  The  mammalian  red  blood  corpuscle  begins 
its  life  as  a  nucleated  cell,  but  later  on  the  nucleus  is  lost. 

The  position  of  the  nucleus  in  the  cell  is  determined  largely  by  physical 
causes,  such  as  surface  tension,  the  position  of  the  vacuoles,  and  the 
relative  density  of  the  cytoplasm  in  different  portions  of  the  cell.  In  a 
non-vacuolated  cell  it  ordinarily  occupies  the  center  of  the  cytoplasmic 
mass,  whereas  in  a  cell  with  vacuoles  it  is  imbedded  in  the  cytoplasm  even 
when  the  latter  is  reduced  to  a  thin  parietal  layer;  it  never  lies  free  in  the 
vacuole.     In  the  Cladophora ceae  (Carter  1919)  it  is  regularly  imbedded, 


77/ A'  NUCLEUS 


61 


Fig.    16. 


at  least  partially,  in  the  chloroplast,  and  this  is  true  even  in  cells  pog  i     - 

ing  a  considerable  amount  of  colorless  cytoplasm.  lis  position  is  also 
related  to  the  functions  of  the  cell:  in 
general  it  lies  in  the  region  character- 
ized by  the  most  active  metabolism. 
For  example,  in  young  growing  root 
hairs  (Fig.  16,  B)  and  pollen  tubes  it 
is  commonly  found  where   elongation  is 

taking  place,  and  in  thickening  epidermal         A    The   thickening  of 
cells  (Fig.  16,  A)  it  frequently,  though    the  inner  wall  of  an  epi- 

.  ,.  .i  n  dermal    cell    of    Scopolia 

not    always,    lies    near   the    wall   upon    atropoides.     H    origin    of 

which  the  thickening  material  is  being    root  hairs  in Pisum  sativum. 

deposited.     This  relation  of  position  to     '  fter    a  l ' 

function   was  emphasized  in  the  works 

of    Haberlandt    (1887)   and    Gerassimow    (1890,    1899,    1901). 

In  form  the  nucleus  is  typically  spherical  or  ellipsoidal,  its 

shape  being    determined    by    a    number    of    physical    factors. 

Under    comparatively    uniform    conditions,    as    obtain  where  a  small 

nucleus  lies  in  a  relatively  large    amount  of  non-vacuolated  cytoplasm, 

a    spherical,  shape  is  assumed  because    of  the   phenomena  of   surface 

tension.     Exceptions  are  often  seen  in  cells  with  specialized  functions. 


Fig.  17. — Unusual  forms  of  nuclei. 
A,  portion  of  nucleus  from  spinning  gland  of  Vanessa  urticce,  Bhowing  irregular  form 
and  finely  divided  state  of  the  chromatin.  {After  Korschelt,  L896.)  />.  Spirostomum 
ambiguum,  with  moniliform  nucleus.  (After  Shin.)  C,  Nucleus  from  Balivary  gland  of 
Chironomus:  the  chromatic  material  exists  as  a  series  of  discs  in  a  convoluted  thread,  which 
ends  in  two  nucleoli.  {After  Balbiani,  1881.)  D,  Choenia  teres,  with  chromatic  granules 
scattered  throughout  the  body.  (After  Oruber,  1884.)  /••'.  Nucleus  from  root  tip  of  Afar- 
silia,  showing  concentration  of  chromatic  material  in  the  nucleolus.     (After  Berghs,  1909. 

In  the  cells  of  the  spinning  glands   of  Pieris  and   Vanessa  (butterflies 
the  physiological  conditions  result   in  the  assumption  of  very  irregular 
forms  whereby  the  nuclear  surface  is  considerably  increased  (Fig.  17,  A). 
Nuclei  seem  rather  commonly  to  undergo  amoeboid  changes  in  shape; 


62  INTRODUCTION  TO  CYTOLOGY 

such  active  movement  can  be  directly  observed  in  the  nucleus  of  the 
living  cycad  spermatozoid.  In  the  long,  narrow  cells  of  vascular  bundles 
the  nuclei,  which  arc  not  free  to  grow  in  all  dimensions,  come  to  be 
correspondingly  elongated.  The  nucleus  may  also  be  passivehT  forced 
into  very  irregular  shapes  by  the  dense  accumulation  of  starch  grains 
and  the  diminution  in  the  amount  of  cytoplasm,  as  in  the  endosperm 
cells  of  maize.  Tn  Stentor  and  Spirostomuni  the  nucleus  has  the  form 
of  a  string  of  beads  (Fig.  17,  B). 

In  size  the  nucleus  shows  a  wide  variation,  ranging  in  plants  from 
the  extremely  minute  nucleus  of  Mucor,  1/j,  or  less  in  diameter,  to  the 
relatively  gigantic  nucleus  of  the  Dioon  egg,  with  a  diameter  of  600//,. 
A  similar  range  is  seen  in  animal  nuclei.  Although  the  nuclei  of  the 
fungi  arc  characterized  by  small  size,  most  of  them  being  less  than  o/j. 
in  diameter,  they  may  grow  to  a  large  size  at  certain  stages.  The  primary 
nucleus  of  Synchytrium,  for  instance,  reaches  a  diameter  of  over  60//.. 
The  majority  of  nuclei,  however,  fall  between  5m  and  25ju.  In  spite  of 
the  wide  range  in  the  size  of  nuclei  of  different  organisms,  in  a  given 
tissue  it  is  comparatively  uniform. 

With  respect  to  the  physical  nature  of  the  nucleus  as  a  whole,  the 
researches  of  Kite  (1913)  and  Chambers  (1914,  1917)  have  shown  that  it 
ordinarily  consists  at  least  in  part  of  a  gel  of  higher  viscosity  than  the 
cytoplasm,  often  being  so  firm  that  it  can  easily  be  handled  without 
injury  by  means  of  the  dissecting  instrument.  This  obviously  would  be 
impossible  were  the  nucleus  merely  a  watery  droplet  or  vesicle  in  the  cyto- 
plasm. The  germinal  vesicle  (nucleus)  of  the  animal  egg  Chambers 
(1917)  finds  to  be  a  sol  droplet  with  a  gel  membrane;  if  it  is  pinched  in  two 
by  the  dissecting  instrument  the  two  halves  will  reunite  if  they  come  in 
contact. 

The  chemical  nature  of  the  nucleus  has  been  dealt  with  in  the  preced- 
ing chapter.  With  regard  to  its  electrical  properties,  the  nucleus  is 
apparently  negative  to  the  cytoplasm.  R.  S.  Lillie  (1903)  found  that 
free  nuclei  and  the  heads  of  spermatozoa,  which  are  almost  entirely 
nuclear  material,  pass  to  the  anode  in  an  isotonic  cane  sugar  solution, 
whereas  cells  rich  in  cytoplasm,  such  as  large  leucocytes,  pass  to  the 
cathode.     These  results  have  been  confirmed  by  Hardy  (1913). 

Nucleoplasmic  Ratio. — Of  more  importance  than  the  absolute  size 
of  the  nucleus  is  the  relation  of  its  volume  to  that  of  the  cytoplasm — the 
so-called  nucleoplasmic  or  Kernplasma  relation.  Many  years  ago  it  was 
held  by  Sachs  (1892,  1893,  1895)  and  by  Strasburger  (1893)  that  the  size 
of  a  meristematic  cell  in  a  plant,  owing  to  a  supposed  limitation  of  the 
sphere  of  influence  of  the  nucleus,  maintains  a  very  definite  relation  to 
the  size  of  its  nucleus.  This  conception  has  recently  been  emphasized 
anew  by  Winkler  (1916),  and  parallel  views  have  been  expressed  by 
several  zoologists  (e.g.,  Hegner  on  Arcella,  1919).     In  the  case  of  certain 


Til E  NUCLEUS 


terminal  meristems  of  plants  such  a  rule  may  well  hold  true  within 
limits,  but  the  condition  reported  by  Bailey  (1920)  in  the  lateral  meristem 
(cambium)  shows  clearly  Unit  it  cannot  have  universal  application. 
The  cambial  initials  may  vary  enormously  in  size  with  no  corresponding 
variation  in  the  size  of  1  heir  nuclei:  two  such  initials,  one  of  them  having 
many  hundreds  of  times  the  volume  of  the  other,  may  possess  nuclei 
of  approximately  equal  size. 

The  nucleoplasmic  ratio  has  figured  prominently  in  discussions  of 
the  problem  of  senescence.  R.  Hertwig  in  1889  advanced  the  theory 
that  senescence  and  natural  death  are  associated  with  an  increase  in  the 
relative  size  of  the  nucleus.  He  later  asserted  (1903, 1908)  that  the  nucleo- 
plasmic relation  is  self-regulatory  within  certain  limits  for  each  kind  of 
cell,  exercising  thereby  a  control  over  many  cell  activities,  including  cell- 
division.  Minot  (1891,  1908,  1913),  on  the  contrary,  believed  that  the 
increase  in  the  relative  volume  of  the  cytoplasm,  in  addition  to  its  differ- 
entiation, is  a  fundamental  factor  in  senescence  and  death.  Conklio 
(1912),  as  a  result  of  his  work  on  Crepidula,  denied  the  existence  of  a 
constant  and  self-regulatory  nucleoplasmic  relation,  holding  rather  that 
changes  in  this  relation  are  not  causes  of  such  cell  activities  as  cell- 
division,  but  are  results  of  the  metabolic  processes  by  which  such  cell 
activities  are  brought  about.  Child  (1915)  points  out  that  in  most 
animal  tissues  there  is  an  increase  in  the  relative  amount  of  cytoplasm 
during  senescence,  whereas  in  plants,  although  the  cell  enlarges  through 
vacuolation,  the  relative  volume  of  cytoplasm  often  does  not  increase. 
He  therefore  concludes  that  the  nucleoplasmic  relation  cannot  be  regarded 
as  a  universal  factor  in  senescence;  it  is  rather  an  indication  of  the  kind 
and  rate  of  metabolism.  The  differentiation  of  the  cytoplasm,  apart 
from  its  mere  change  in  volume,  Child,  with  many  other  workers  <  Minot . 
Delage,  Jennings,  etc.),  regards  as  a  matter  of  the  greatest  importance  in 
senescence.     Further  discussion  of  this  subject  is  deferred  to  ( )hapter  VII. 

Not  only  has  it  been  held  that  there  is  a  certain  relation  between  the 
mass  of  the  nucleus  and  that  of  the  cytoplasm,  whatever  the  significance 
of  this  relation  may  be,  but  there  also  seems  to  be  a  size  relationship 
between  the  nucleus  and  its  contained  chromosomes.  In  L896  Boveri 
showed  that  the  size  of  the  nuclei  in  merogonic  echinoderm  larvse  (s 
p.  325)  is  dependent  upon  the  number  of  chromosomes  each  contain-. 
In  a  more  extended  study  (1905)  he  demonstrated  that  it  is  the  surface 
of  the  nucleus  that  is  proportional  to  the  chromosome  number,  and  also 
that  the  size  of  the  cell  is  proportional  to  both.  ( rates  1 190!)),  however, 
adduced  evidence  to  show  that  this  rule  is  by  no  means  universal. 

Structure. — Having  reviewed  the  general  features  of  the  nucleus  as 
a  whole,  we  may  next  give  attention  to  its  structure,  as  -ecu  in  typical 
cases. 

The  nucleus  is  bounded  by  a  distinct  nuclear  membraru  .     The  nature 


64  INTRODUCTION  TO  CYTOLOGY 

of  this  membrane  has  been  a  subject  of  much  controversy.  Some  have 
regarded  it  as  a  precipitation  membrane  laid  down  when  the  newly 
formed  karyolymph  comes  in  contact  with  the  cytoplasm  at  the  time  the 
daughter  nuclei  are  reconstructed  during  the  closing  phases  of  mitosis, 
while  others  (Lawson  1903)  have  interpreted  it  as  merely  a  denser  limit- 
ing layer  of  the  -cytoplasm.  The  above  cited  work  of  Kite  and  Chambers, 
however,  leaves  no  doubt  that  the  membrane  is  a  definite  morphological 
structure  belonging  to  the  nucleus:  although  it  is  at  times  very  delicate, 
it  remains  intact  when  the  nucleus  is  pushed  and  pulled  about  by  the 
dissecting  instrument,  and  is  thrown  into  folds  when  the  karyolymph  is 
withdrawn  with  a  pipette. 

Within  the  nuclear  membrane  is  a  series  of  gels  of  varying  consistency. 
The  nuclear  sap,  or  karyolymph,  is  a  highly  transparent  substance  which 
is  generally  looked  upon  as  homogeneous,  although  it  has  been  thought 
by  some  workers  (Reinke  1894)  to  be  made  up  of  large,  pale  "cedamatin 
granules."  It  may  be  in  the  sol  or  gel  state.  Imbedded  in  the  karyo- 
lymph is  a  network  or  reticulum,  which  may  be  relatively  uniform  through- 
out the  nucleus  or  only  fragmentary  and  incomplete.  It  is  usually  said 
to  be  composed  of  a  gel  substance  known  as  achromatin  (Flemming  1879) 
or  linin  (Schwarz  1887).  Supported  on  the  linin  reticulum  is  the  chroma- 
tin (Flemming  1879).  This  highly  stainable  substance  may  exist  in  the 
form  of  small  granules  or  droplets  at  the  nodes  of  the  reticulum,  or 
apparently  in  many  nuclei  as  a  fluid  thin  enough  to  distribute  itself  more 
or  less  uniformly  throughout  the  achromatic  substance.  In  the  latter 
case  the  whole  reticulum  appears  to  be  composed  of  a  single  unevenly 
stained  material,  careful  examination  showing  the  " chromatic  granules'1 
and  "  achromatic  support "  to  be  its  thicker  and  finer  portions  respectively 
(Fig.  51)  (Gregoire  and  Wyagerts  1903;  Gregoire  1906;  Sharp  1913, 
1920).  According  to  Kite  (1913)  the  granules  in  the  living  nucleus  con- 
sist of  a  very  concentrated  gel,  the  supporting  reticulum  of  a  somewhat 
more  dilute  but  not  at  all  fibrous  gel,  and  the  karyolymph  of  a  gel  which 
is  the  most  dilute  of  all. 

Heidenhain  (1894)  found  imbedded  in  the  colorless  linin  net  two  sorts 
of  chromatin  in  the  form  of  granules:  oxychromatin,  consisting  largely  of 
plastin,  poor  in  phosphorus,  and  staining  with  the  acid  dyes;  and  basi- 
chromatin,  composed  mainly  of  nuclein,  rich  in  phosphorus,  and  staining 
with  the  basic  dyes.  These  two  forms  of  chromatin  apparently  may 
change  into  each  other  by  the  addition  or  loss  of  phosphorus.  The  peri- 
odic changes  in  the  staining  reactions  of  many  nuclei  therefore  indicate 
changes  in  the  chemical  composition  of  the  chromatin,  and  these  in  turn 
point  to  the  intimate  association  of  the  nucleus  with  the  periodic  physi- 
ological processes  of  the  cell.  As  used  by  many  writers  the  term  oxy- 
chromatin includes  also  the  linin,  so  that  in  much  cytological  literature 
linin  and  oxychromatin  are  more  or  less  interchangeable  terms,  while 


THE  NUCLEUS  65 

"chromatin"  refers  to  the  basichromatin.  Oxychromatin  appears  to  be 
closely  similar  in  composition  to  the  achromatic  structures  in  the  cyto- 
plasm, such  as  spindle4  fibers  and  centrosomes.  The  prominent  place 
occupied  by  the  nucleus  in  cytology  is  due  in  large  measure  to  the  con- 
spicuous behavior  of  its  chromatic  substance  at  the  time  of  cell-divisioo 
and  fertilization,  topics  which  are  to  receive  detailed  consideration  in 
subsequent  chapters. 

In  many  nuclei  basichromatin  accumulates  at  certain  points  in  the 
reticulum,  forming  karyosomes,  also  called  "net  knots'  and  chromatin 
nucleoli.  These  seem  to  be  masses  of  surplus  chromatin  elaborated  by 
the  nucleus  during  the  resting  phase  or  in  some  cases  chromatin  which 
has  flowed  to  these  points  from  the  other  parts  of  the  reticulum.  During 
the  next  mitosis  they  are  distributed  with  the  rest  of  the  chromatin,  ks 
Rosen  (1892)  long  ago  showed  by  his  studies  of  their  staining  reactions, 
they  differ  decidedly  in  composition  from  true  nucleoli,  although  they 
may  closely  resemble  the  latter  after  treatment  with  certain  stains  (iron- 
alum-hsematoxylin) . 

One  or  more  true  nucleoli,  or  plasmosomes  (Ogata  1883),  are  usually 
present  in  the  nucleus.  A  single  nucleolus  is  probably  characteristic  of 
most  nuclei;  there  are  rarely  many,  and  in  some  cases  there  is  none.  The 
nucleolus  may  be  in  close  organic  connection  with  the  nuclear  reticulum 
or  it  may  lie  entirely  apart  from  it.  In  composition  it  consists  largely  of 
such  oxychromatic  substances  as  plastin  and  pyrenin,  or  of  nuclei n  well 
saturated  with  protein  (Zacharias).  It  usually  stains  with  the  acid 
dyes:  by  a  proper  selection  of  stains  it  may,  therefore,  be  distinguished 
from  the  karyosomes,  which,  being  composed  of  basichromatin,  take  the 
basic  dyes  as  a  general  rule.  In  structure  the  nucleolus  may  appear  to 
be  homogeneous  throughout,  like  an  oil  globule;  in  other  cases  it  has  an 
outer  envelope  of  different  consistency  and  staining  reaction.  Very  often 
vacuoles,  occasionally  containing  granules,  are  presenl  in  the  interior. 
Crystalloid  bodies  are  also  frequently  observed  in  the  nucleolus  (Digby 
on  Galtonia,  1910;  Reed  on  Allium,  1914;  Kuwada  on  Zca ,  1919).  In 
the  epithelial  cells  of  the  frog  intestine  Carleton  (1920)  finds  one  or  more 
intranucleolar  bodies  which  he  calls  "nucleolini."  These  appear  to 
divide  and  pass  to  the  daughter  cells  at  the  time  of  mitosis,  and  may 
possibly  initiate  the  formation  of  new  nucleoli  in  the  daughter  nuclei, 
Montgomery  (1899)1  concluded  that  the  nucleolus  grows  in  size  by  the 
apposition  of  smaller  particles  of  nucleolar  material  on  its  surface,  and 
by  the  intussusception  of  vacuolar  substance  arising  outside  the  nucleolus. 

Function  of  Nucleolus. — Various  opinions  have  been  entertained  re- 
garding the  function  of  the  nucleolus.  By  many  workers  it  has  been 
looked  upon  as  chiefly  a  passive  by-produd  of  no  furl  her  use  in  the  life 

1  An  exhaustive  review  of  the  literature  dealing  with  the  nucleolus  up  to   1890 

is  given  in  this  paper. 
5 


66  INTRODUCTION  TO  CYTOLOGY 

of  the  cell  (Haecker).  Strasburger  (1895,  1897),  who  observed  the  dis- 
appearance of  the  nucleolus  at  about  the  time  the  spindle  fibers  appear 
during  the  prophases  of  mitosis,  concluded  that  it  is  a  mass  of  reserve 
kinoplasm  which  gives  rise  indirectly  to  the  achromatic  figure.  While 
some  have  agreed  in  the  main  with  this  conclusion,  many  have  denied 
the  relationship  of  nucleolus  and  spindle,  contending  that  the  former  is 
rather  a  reserve  constituent  for  the  linin  reticulum  (Eisen  1900)  or  the 
chromatin  (Schurhoff  1918).  Frequently  the  bulk  of  the  basichrom- 
atic  material  of  the  nucleus  is  lodged  in  the  nucleolus  at  certain  stages. 
In  the  somatic  nuclei  of  Marsilia  (Fig.  17,  E),  for  example,  Berghs  (1909) 
shows  that  it  is  transferred  to  the  nucleolus  during  the  telophases  of 
mitosis,  and  returned  to  the  reticulum  in  the  following  prophases.  This 
phenomenon,  which  has  an  important  bearing  on  the  role  of  the  chrom- 
atin  and  the  individuality  of  the  chromosomes,  will  be  referred  to  again 
in  Chapter  VIII. 

In  many  cells,  as  shown  by  the  work  of  the  zoologists  the  nucleolus 
appears  to  be  concerned  in  the  elaboration  of  secretion  and  storage  prod- 
utts.  In  the  eggs  of  certain  animals  Macallum  (1890)  showed  that 
the  nucleolar  material,  which  appears  to  differentiate  from  the  chrom- 
atin, passes  into  the  cytoplasm  and  there  combines  with  another 
substance  to  form  the  yolk  globules.  In  the  cells  of  the  pancreas  he 
further  found  that  material  often  present  in  the  form  of  nucleoli  func- 
tions in  a  similar  manner  in  the  production  of  zymogen.  Many  other 
observations  of  this  general  nature  have  been  reported.  In  the  silk- 
gland  cells  of  certain  insects  it  has  recently  been  shown  by  Nakahara 
(1917)  that  some  of  the  nucleoli,  which  may  originally  be  passive  by- 
products, later  pass  into  the  cytoplasm  and  contribute  to  the  formation 
of  the  secretion  products.  An  extreme  view  of  the  importance  of  the 
nucleolus  is  that  of  Derschau  (1914),  who  regards  the  nucleolus  as  the 
real  center  of  the  life  of  the  cell.  Granules  of  oxychromatin,  he  asserts, 
pass  out  from  the  nucleolus  through  the  cytoplasm  in  the  form  of  chon- 
driosomes,  carrying  basichromatin  as  a  building  material  to  the  places 
where  it  is  required. 

It  is  highly  probable  that  the  nucleolus  has  various  functions  in  dif- 
ferent cells,  but  in  general  we  majr  conclude  that  it  is  a  mass  of  accumu- 
lated material  which  is  usually,  though  not  always,  utilized  in  the 
metabolic  processes  of  the  nucleus 

The  Nuclei  of  Bacteria  and  Other  Protista. — The  question  of  the 
nucleus  in  bacteria  is  one  that  it  appears  to  be  particularly  difficult 
to  settle  satisfactorily.  This  is  due  not  only  to  the  minute  size  of  these 
'organisms,  which  makes  special  methods  necessary  and  observation  very 
difficult,  but  also  to  the  fact  that  a  variety  of  conditions  seems  to  be 
present  in  the  group.  That  the  bacterial  cell  is  devoid  of  a  nucleus  has 
been  held  by  several  investigators  including  Fischer  (1894,  1897,  1899, 


77/ A'  NUCLEI  S  67 

1903),  who  looked  upon  the  observed  granules  as  reserve  materials 
rather  than  nuclear  substance.  Migula  (1894,  1897,  L904)  regarded  the 
existence  of  nuclei  in  bacteria  as  very  doubtful.  The  majority  of  workers, 
on  the  contrary,  have  held  thai  a  nucleus  or  at  leasl  nuclear  material  is 
present  in  some  form.  The  most  striking  view  is  thai  which  regards  the 
whole  cell  in  some  cases  as  a  uaked  nucleus  (Htippe  L886;  Zettnow  L891, 
1897,  1899;  Ruzicka  1908,  1909;  and,  in  the  case  of  small  bacteria, 
Biitschli  1890,  1892,  1896,  1902).  The  evidence  advanced  in  supporl  of 
this  hypothesis,  however   is  of  very  doubtful  value. 

In  many  bacteria,  particularly  the  larger  forms,  there  is  presenl  ;i 
granular  substance  which  has  certain  characteristics  of  chromatin,  and 
which  in  some  species  exists  as  a  single  well  defined  mass.  The  "central 
body"  of  the  sulphur  bacterium  Btitschh  regarded  as  the  homologue  of  a 
nucleus,  the  peripheral  portion  of  the  cell  being  cytoplasm.  In  a  careful 
study  of  the  entire  life  cycle  of  Bacillus  Biltschlii  Schaudin  I  1902)  found 
that,  the  chromatic  material  present  during  most  of  the  cycle  as  chromidia 
unites  at  certain  stages  to  form  peculiar  spiral  figures;  in  the  spores  it 
takes  the  form  of  dense  masses.  Such  scattered  chromidia  and  'spiral 
filament  nuclei"  were  also  observed  by  Guilliermond  (1908,  1909),  who 
has  given  a  review  of  thesubject  (1907).  Nakanishi  (1901 ),  who  employed 
both  intra-vitam  methods  and  fixed  material,  reported  the  presence  of 
nuclei  in  the  vegetative  cells  and  spores  of  a  number  of  speci<  3. 

The  nucleus  of  the  large  Bacterium  gammari  was  studied  by  Vejdow- 
sky  (1900),  who  in  1904  described  its  division  by  mitosis.  Mend  L904, 
1905,  1907,  1909)  demonstrated  by  careful  methods  the  nuclei  in  many 
species  and  also  reported  mitotic  division  in  Bacterium  gammari.  I  )oub1 
concerning  the  systematic  position  of  this  form,  however,  has  been  raised 
by  some  investigators,  who  think  it  not  improbable  that  it  is  a  yeast- 
like fungus  rather  than  a  bacterium. 

Dobell  (1908,  1909,  1911),  whose  review  of  the  subject  has  been  of 
service  in  the  preparation  of  this  summary,  has  studied  with  much  care 
many  species  of  bacteria  in  their  natural  culture  media.  His  conclusions 
are  summarized  in  the  following  quotation  (1911): 

"All  bacteria  which  have  been  adequately  investigated  are  like  all 
other  Protista — nucleate  colls. 

"The  form  of  the  nucleus  is  variable,  not  only  in  different  bacteria, 
but  also  at  different  periods  in  the  life  cycle  of  the  same  species. 

"The  nucleus  may  be  in  the  form  of  a  discrete  system  of  granules 
(chromidia);  in  the  form  of  a  filament  of  various  configuration;  in  the 
form  of  one  or  more  relatively  large  aggregated  masses  of  auclear  sub- 
stance; in  the  form  of  a  system  of  irregularly  branched  or  bent  short 
strands,  rods,  or  networks  and  probably  also  in  the  vesicular  form  char- 
acteristic of  the  nuclei  of  many  animals,  plants,  and  protists. 

"There  is  no  evidence  that  enucleate  bacteria  exist." 


68  INTRODUCTION  TO  CYTOLOGY 

The  apparent  discrepancy  between  this  view  of  bacterial  organization 
and  that  of  Minchin,  stated  below,  will  be  seen  to  be  largely  a  matter  of 
terminology. 

It  is  therefore  among  the  Proitsta  that  the  widest  departures  from 
the  usual  type  of  nuclear  structure  are  found,  certain  of  them  in  all  prob- 
ability representing  relatively  primitive  stages  in  the  evolution  of  the 
true  nucleus.  Such  an  interpretation  is  evidently  to  be  placed  upon  the 
"distributed  nuclei1  seen  in  certain  bacteria,  protozoans,  flagellates, 
and  Cyanophycere  (p.  202),  which  consist  of  granules  of  a  material  akin 
to  chromatin  scattered  throughout  the  cell,  sometimes  with  a  limiting 
membrane  of  some  sort  but  often  with  none.  It  is  doubtful  if  granules 
scattered  with  no  definite  limitations  throughout  the  cell,  as  in  Chcenia 
teres  (Fig.  17,  D)  or  Chroococcus  turgidus  (Fig.  72,  A),  should  be  spoken 
of  collectively  as  a  nucleus.  As  pointed  out  at  the  beginning  of  this 
chapter,  it  seems  preferable  to  certain  workers  to  limit  the  term  to  those 
chromatic  aggregations  which  actually  have  the  characters  of  a  definitely 
localized  organ.  In  discussing  the  advisability  of  so  restricting  the  appli- 
cation of  the  term  Minchin  (1912,  Chapter  VI)  points  out  that  "the  word 
' chromatin'  connotes  an  essentially  physiological  and  biological  con- 
ception .  .  .  of  a  substance,  far  from  uniform  in  its  chemical  nature, 
which  has  certain  definite  relations  to  the  life  history  and  vital  activities 
of  the  cell.  The  word  'nucleus,'  on  the  other  hand  .  .  .  is  essentially 
a  morphological  conception,  as  of  a  body,  contained  in  the  cell,  which 
exhibits  a  structure  and  organization  of  a  certain  complexity,  and  in 
which  the  essential  constituents,  the  chromatin  particles,  are  distributed, 
lodged,  and  maintained,  in  the  midst  of  achromatinic  elements  which 
exhibit  an  organized  arranegment,  variable  in  different  species,  but  more 
or  less  constant  in  the  corresponding  phases  of  the  same  species."  Ac- 
cording to  this  interpretation  the  term  "nucleus"  would  not  be  applicable 
to  a  mass  of  granules  (chromidia)  scattered  throughout  the  cell.  Minchin 
states  further  that  "  .  .  .as  soon  as  a  mass  or  a  number  of  particles  of 
chromatin  begin  to  concentrate  and  separate  themselves  from  the  sur- 
rounding protoplasm,  with  formation  of  distinct  nuclear  sap  and  ap- 
pearance of  achromatinic  supporting  elements,  we  have  the  beginning 
at  least  of  that  definite  organization  and  structural  complexity  which  is 
the  criterion  of  a  nucleus  as  distinguished  from  a  chromidial  mass." 

Those  Protista  of  the  lower  (bacterial)  grade,  in  which  there  are  only 
scattered  grains  of  chromatic  material,  are  looked  upon  as  " non-cellular '! 
in  organization  by  Minchin,  who  believes  that  from  such  a  primitive 
state  the  "strictly  cellular  grade  of  organization  has  been  evolved  by 
concentration  of  some  or  all  of  the  chromatin  to  form  a  nucleus."  In 
its  simplest  condition  such  a  nucleus  consists  of  one  or  more  chromatin 
granules  in  a  sort  of  vacuole,  and  is  known  as  a  "  protokaryon."  In  other 
cases  the  chromatin  forms  a  single  large  mass  at  the  center  of  the  nucleus 


77/  E  NUCLEUS  69 

("vesicular  nucleus").  Since  "the  chromatic  particles  are  the  only  con- 
stituents of  the  cell  which  maintain  persistently  and  uninterruptedly 
their  existence  throughout  the  whole  life  cycle  of  living  organisms  uni- 
versally," Minchin  (1916)  believes  that  the  earliest  living  things,  which 
he  calls  "  Biococci/'  were  minute  particles  of  a  chromat in-like  substance. 
These  were  the  ancestors  of  the  present  chromatin  grains  and  find  their 
nearest  modern  representatives  in  certain  pathogenic  Chlamydozoa.  Ac- 
cording to  this  view  the  cytoplasm  was  differentiated  later  in  the  evolu- 
tion of  the  cell,  whereas  the  more  general  view  probably  is  that  chromatin 
and  cytoplasm  were  coexistent  as  two  substances  in  cells  from  theearliesl 
known  stages  (Wilson).1 

The  Function  of  the  Nucleus. — It  may  be  said  without  reservation 
that  the  nucleus  dominates  the  morphological  and  physiological  changes 
in  the  cell.  Although  the  type  of  organization  formed  by  a  nucleus  in 
combination  with  cytoplasm  is  required  for  the  carrying  on  of  cell  activity, 
it  is  nevertheless  evident  from  a  huge  mass  of  accumulated  observations 
that  in  the  nucleus  is  to  be  found  the  center  of  control  for  both  the  func- 
tional activities  and  for  cell  reproduction  (cell-division).  Many  years 
ago  Claude  Bernard  (1878)  pointed  out  that  while  the  cytoplasm  is  the 
seat  of  destructive  metabolism,  the  nucleus  is  the  seat  of  constructive 
metabolism,  this  physiological  role  offering  "the  key  to  its  significance  as 
the  organ  of  development,  regeneration,  and  inheritance  "  (Wilson).  The 
inability  of  a  cell  deprived  of  its  nucleus  to  carry  on  synthetic  metabolism 
in  any  complete  manner  has  often  been  noted,  though  such  a  cell  may  not 
perish  for  some  time.  The  mammalian  erythrocyte,  for  example,  loses 
its  nucleus  at  an  early  stage  and  may  continue  to  exist  in  the  enucleate 
state  for  from  15  to  30  days  (Hunter,  Quincke).  Klebs  found  that 
enucleate  cells  of  Spirogyra  may  continue  for  some  time  to  form  starch. 
But  such  cells  are  apparently  unable  to  divide  or  to  increase  their  bulk- 
by  the  elaboration  of  new  cell  substance.  Many  ordinary  act  Lvities,  such 
as  cell  wall  formation  (Townsend  1897;  Gerassimow  1899,  1901),  fail  to 
occur.  From  such  observations  it  is  concluded  that  the  nucleus  is  acc- 
essary for  the  synthetic  processes  associated  with  growth  and  reproduc- 
tion.    This  conclusion  is  supported  by  the  facts  of  regeneration. 

The  role  of  the  nucleus  in  regeneration  was  strikingly  shown  by  the 
well  known  experiments  of  Gruber  (1885)  and  F.  H.  Li  Hie  (1896)  on  SU  ><- 
tor.  This  unicellular  organism,  which  has  a  nucleus  like  a  string  of  beads, 
may  be  cut  into  fragments:  any  fragment  containing  a  portion  of  the 
nucleus  has  the  power  of  regenerating  a  complete  new  animal,  whereas 
enucleate  fragments,  although  they  may  live  for  a  little  time,  undergo  do 
regeneration  and  eventually  perish. 

'For  more  complete  descriptions  of  the  nuclei  of  Protista  the  works  of  Wilson 
(1900)  and  Minchin  (1912)  should  be  consulted.  The  behavior  <>f  such  nuclei  at  the 
time  of  coil-division  is  briefly  described  in  Chapter  X  of  tliis  book. 


70  INTRODUCTION  TO  CYTOLOGY 

A  number  of  biologists  (Gruber  1886,  Hertwig  1898,  Heidenhain  1894, 
Henneguy  1896,  Conklin  1902)  concluded  that  in  general  the  chromo- 
somes (basichromatm)  are  concerned  chiefly  with  differentiation  and 
regulation,  while  the  achromatin  (oxychromatin)  has  to  do  with  metabo- 
lism (Conklin  1917).  Metabolism  is  in  reality  a  great  complex  of 
reactions:  the  reactions  are  not  independent  of  one  another  but  are  closely 
correlated,  and  thus  constitute  an  intricately  adjusted  reaction  system. 
Among  these  many  reactions,  according  to  modern  physiology,  the  most 
important  is  oxidation,  for  the  energy  utilized  by  the  organism  is  derived 
immediately  from  the  union  of  protoplasm  or  of  its  constituent  elements 
with  nwgen.  Oxidation  has  been  called  the  "independent  variable' 
(Loeb  and  Wasteneys  1911)  upon  which  the  other  reactions  largely 
depend:  oxidation  is  the  dominant  factor  in  cell  activity,  and  it  is  there- 
fore of  the  greatest  importance  to  understand  as  well  as  possible  the  rela- 
tion of  the  parts  of  the  cell  to  this  process. 

Following  the  experiments  of  Spitzer  (1897),  who  observed  thatnucleo- 
proteins  extracted  from  certain  animal  tissues  have  the  same  oxidizing 
power  as  the  tissues  themselves,  it  was  advocated  by  Loeb  (1899)  that  the 
nucleus  is  the  center  of  oxidation  in  the  cell.  Loeb  pointed  out  that  this 
would  explain  the  inability  of  enucleated  cell-fragments  to  undergo 
regeneration.  This  conclusion  was  supported  by  R.  S.  Lillie  (1903),  who 
later  (1913)  showed  that  rapid  oxidation  occurs  both  at  the  surface  of  the 
cell  and  at  the  surface  of  the  nucleus,  and  also  by  Mathews  (1915).  Other 
workers  (Wherry  1913,  Schultze  1913,  Reed  1915)  however,  have  failed 
to  agree.  Osterhout  (1917),  who  briefly  summarizes  the  subject,  found 
that  "injury  produces  in  the  leaf-cells  of  the  Indian  Pipe  (Monotropa 
uni flora)  a  darkening  which  is  due  to  oxidation.  The  oxidation  is  much 
more  rapid  in  the  nucleus  than  in  the  cytoplasm  and  the  facts  indicate 
that  this  is  also  the  case  with  the  oxidation  of  the  uninjured  cell." 

The  role  of  the  nucleus  in  development  and  inheritance,  which  has 
been  a  subject  of  so  much  discussion  in  recent  years,  will  be  dealt  with 
in  later  special  chapters  (XIV-XVIII),  after  the  behavior  of  the  nucleus 
in  somatic  cell-division,  maturation,  and  fertilization  has  been  described. 

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Zettnow,  E.     L891.     Ueber  den  Bau  den  Bakterien.     Centr.  Bakt.  10:  690. 
1897.     Ueber  den  Bau  der  Grossen  Spirillum.     Zeit.  Hyg.  24:  72. 
1899,    1900.     Romanowski's    Farbung    bei    Bakterien.     [bid.   30:    I.   and    Centr 

Bakt.  1  27:  803. 
1908.     Ueber  Schwellengrebel's  Chromatinbander  in  Spirillum  minimis.     Centr 
Bakt.  1  46:  193. 
Zimmermann,  A.     1893-1894.     Sammel-Referate.     5,   7.   8.     Beih.   Hot.   Centr.   3: 
333-342,  401-430;  4:  81-89.     (No.  7  reviews  153  papers  on  nuclei  of  various 
plant  "roups.) 


CHAPTER  V 

THE  CENTROSOME  AND  THE  BLEPHAROPLAST 

THE  CENTROSOME 

For  a  full  description  of  the  morphology  and  behavior  of  the  centro- 
some,  based  upon  the  large  amount  of  work  done  on  animal  cells  prior  to 
1900,  reference  should  be  made  to  Wilson's  book  on  the  cell.  In  the 
present  account  attention  will  be  devoted  mainly  to  centrosome  structures 
in  plants.  The  centrosomes  of  animal  cells  will  be  described  only  in 
general  terms,  their  role  in  cell-division  being  dealt  with  in  later  chapters. 

Occurrence  and  General  Characters. — The  centrosome  is  an  organ 
which  is  characteristic  chiefly  of  the  cells  of  animals :  in  the  great  majority 
of  these  cells  it  has  been  found,  at  least  during  certain  stages.  In  plants 
centrosomes  are  limited  to  the  cells  of  algae  and  fungi  and  the  spermato- 
genous  cells  of  certain  bryophytes  and  pteridophytes.  If  the  blepharo- 
plast  be  regarded  as  a  modified  centrosome,  a  question  which  will  be 
discussed  further  on,  all  motile  cells  (spermatozoids)  of  bryophytes, 
pteridophytes,  and  gymnosperms  must  be  looked  upon  as  possessing 
centrosomes.  During  the  last  decade  of  the  nineteenth  century  several 
botanists  reported  the  presence  of  centrosomes  in  the  cells  of  a  number  of 
angiosperms,  but  these  cases  have  all  failed  to  stand  the  test  of  subsequent 
more  critical  research.1 

It  is  scarcely  possible  to  give  a  description  which  will  apply  to  all 
centrosomes,  since  to  any  rule  there  are  apparently  exceptions.  The 
'typical"  centrosome,  as  seen  in  animal  cells,  is  a  very  minute  granule, 
which  stains  intensely  with  certain  dyes.  It  is  usually  situated  in  the 
cytoplasm,  but  in  some  cases  it  is  found  within  the  nucleus  (Fig.  18). 
It  commonly  lies  in  a  more  or  less  hyaline  mass  of  material,  called  the 
centrosphere  (Strasburger  1892),  attraction  sphere  (van  Beneden  1883), 
astrosphere  (Fol  1891),  or  hyaloplasm  sphere  (Wilson  1901). 2  This  centro- 
sphere may  often  show  two  or  more  concentric  zones  differing  somewhat 
in  structure  and  appearance  (Fig.  60).  At  certain  stages,  especially 
during  nuclear  division,  the  centrosome  becomes  the  focus  of  a  system  of 
delicate  rays  known  collectively  as  the  aster  (Fol  1877).     The  aster  will 

1  For  a  review  of  these  cases  see  Koernicke  (1903,  1906). 

2  There  has  been  much   confusion  in    the   application   these   terms.     (See  Wilson 
1900,  p.  324.) 

76 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST 


i  i 


receive  consideration  in  the  chapter  on   the  achromatic  figure.1     Very 
often  there  arc  two  centrosomes  lying  side  by  side  in  the  centrosphere, 


Fig.  18. — Centrosomes  in  animal  cells. 
A,  attraction  sphere  above  nucleus  in  spermatocyte  of  Salamarulm.      (After  Rawitz; 
see  also  Fig.  59.)     B-F,  intranuclear  centrosome  in  spermatocyte  of  Ascaris  megaloceptiala 
and  its  behavior  during  the  prophases  of  mitosis;  c,  centrosphere;  chr,  chromosome  tetrad. 
(After  Brauer,  1893.) 

the  two  having  arisen  by  the  division  of  one,  apparently  in  preparation 
for  the  next  cell-division  (Fig.  19).  Von  Winiwarter  (1912)  noticed 
that  in  interstitial  testicular  cells,  which  may 
have  one,  two,  or  four  nuclei,  there  are  re- 
spectively two,  four,  and  eight  rod-like  cen- 
trosomes lying  in  midst  of  a  granular  mass 
("idiosome").  In  some  cells  there  may  be  a 
larger  number  of  smaller  "centrioles"  rather 
then  one  centrosome,  and  occasionally  there  are 
one  or  more  concentric  series  of  granules  about 
the  central  centrosome.  Several  such  types 
described  by  various  writers  are  shown  in 
Wilson's  Fig.  152.  It  is  questionable  how  far 
these  are  normal  appearances,  for  Chambers 
(1917)  asserts  that  several  of  them  may  be  pro- 
duced in  the  animal  egg  by  subjecting  the  latter 
to  abnormal  environmental  conditions. 

Individuality. — The  centrosome  was  dis- 
covered and  described  by  Flemming  (1875) 
and  independently  by  van  Beneden  (1870).  In 
1887  van  Beneden  and  Boveri,  as  a  result  of 
their  researches  on  the  thread-worm,  Ascaris 
megalocephala,  independently  concluded  that  the 
centrosome,  like  the  nucleus,  is  a  permanent  cell 
organ  maintaining  its  individuality  throughout 

1  Because  of  the  relation  of  the  centrosome  to  the  achromatic  figure  it  will  be 
necessary  to  make  constant  reference  to  the  latter.  Chapter  IX  Bhould  be  consulted 
in  this  connection. 


■ 

A 


Fii 


in 


l'.i.     ( Jentrosomea 
epithelial  cells. 

from  cornea  of  mon- 
key. />'.  from  gastric  gland 
of    man.      (After    Zimn 

mm 


.1 


78 


INTRODUCTION   TO  CYTOLOGY 


successive  eell  generations.  They  observed  that,  prior  to  cell-division, 
the  centrosome  divides  to  form  two  daughter  centrosomes,  which  move 
apart  to  opposite  sides  of  the  cell  and  form  the  poles  between  which  the 
mitotic  figure  is  established;  and  further,  that  after  cell-division  is 
completed  the  centrosome  included  in  each  daughter  cell  does  not 
disappear,  but  remains  visible  in  the  cytoplasm  through  the  ensuing- 
resting  stage.  Because  of  this  striking  behavior  at  the  time  of  cell- 
division  (see  further  p.  177)  the  centrosome  soon  came  to  be  known  as 
"the  dynamic  center  of  the  cell." 

The  above  facts  seemed  to  constitute  ample  ground  for  the  conception 
of  the  centrosome  as  a  permanent  cell  organ,  but  many  obstacles  have 
been  found  in  the  way  of  its  acceptance  as  a  theory  of  universal  applica- 
tion.    At  certain  stages  in  the  history  of  many  animal  cells  its  presence 


-'■ ■-■.- ...  \  ■•;;  —    ^'-4|$8  OxO.- 


Fig.  20. — Artificial  cytasters  in  the  egg  of  Arbacia.      (After  Morgan,  1899.) 

cannot  be  demonstrated,  and  it  is  entirely  absent  from  the  cells  of  higher 
plants.  Furthermore,  Mead  (1898)  and  Morgan  (1896.  1898)  found  that 
the  formation  of  centrosomes  with  asters  may  be  induced  in  the  eggs  of 
certain  animals  by  artifical  means  (treatment  with  NaCl  and  MgCl2 
solutions)  (Fig.  20),  and  it  has  been  claimed  that  centrosomes  so  formed 
may  function  normally  in  the  ensuing  division  (cleavage)  of  the  egg. 
Contrary  to  the  opinion  of  Boveri  (1901),  Wilson  (1901)  regarded  such 
"artificial  cytasters"  as  true  asters  with  true  centrosomes.  Conklin 
(1912),  however/contends  that  they  do  not  function  in  mitosis.  It  is 
probable  that  no  single  conclusion  can  be  drawn  concerning  this  matter 
which  will  apply  to  all  cases.  There  seems  to  be  good  evidence  for  the 
view  that  the  centrosome  in  some  tissues  exists  as  a  permanent  cell  organ, 
dividing  at  each  mitosis  and  remaining  visible  through  the  resting  stages, 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST 


79 


at  least  for  a  number  of  cell  generations.  In  other  cases  n  disappears  al 
the  close  of  mitosis,  a  new  one  being  apparently  formed  just  before  the 
next  mitosis.  The  fact  that  the  formation  of  centrosomes  may  be 
brought  about  by  artificial  means  suggests  thai  the  regular  appearance  of 
the  centrosome  in  successive  mitoses  is  closely  associated  with  regularly 
recurring  physiological  conditions  in  the  cell;  and  thai  its  presence  in 
successive  cell-divisions  does  not  require  an  uninterrupted  morphological 
continuity  through  the  intervening  stages.  Its  constant  presence  in  some 
tissues  probably  indicates  the  continuity  of  some  physiological  function. 
Centrosomes  in  Algae.1 — One  of  the  earliest  known  centrosomes  in 
plants  was  that  of  the  diatom  Surirella,  discovered  by  Smith  (1886  7 
and  Butschli,  and  fully  described  by  Lauferborn  (1896)  and  Karsten 
(1900).  It  lies  near  the  nucleus,  becomes  surrounded  by  radiations,  and 
divides  to  form  the  central  spindle  of  the  mitotic  figure  in  a  very 
peculiar  manner. 


• 


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t  A' 


B 


•  V1    - 


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■'•    • 


Fig.  21. — Centrosomes  in  algae. 

.1.  Stypocaulon.      (After  Swingle,   1897.)      H,  Stypocaulon.      (After  Escoyez,    1909        I 
centrosphere-like    bodies   in    Polysiphonia.     (After    Yamanouchi,    L906.)      1>     /•.'.    Dictyota 
dichotoma.     (After  Mottier,  L900.) 

Centrosomes  in  the  Sphacelariaceae  have  been  described  by  I  [umphrey 
(1894),  Swingle  (1897),  Strasburger  (1900),  and  Escoyez  (1909).  In  the 
vegetative  cells  of  Sphacelaria,  according  to  Strasburger,  the  centrosome 
is  situated  in  a  cent  rosphere  at  t  he  locus  of  an  aster.  Previous  t<»  mitosis 
it  divides  into  two  which  take  up  positions  at  opposite  poles  of  tin- 
spindle.  In  Stypocaulon  (Swingle)  essentially  the  same  condition  exists 
(Fig.  21).  Escoyez  later  concluded,  however,  thai  the  asters  of  Stypocau- 
lon are  formed  independently  rather  than  by  division,  and  thai  the 
central  corpusclse . are  probably  not  true  centrosomes,  but  cytoplasmic 
microsomes. 

1  'This  review  of  plan  I  cent  rosomes  and  also  thai  of  the  blepharoplasl  in  subsequent 
pages  are  based  upon  similar  reviews  given  by  the  author  in  liis  paper  on  Spermato- 
genesis in  Equisetum  (1912). 


80  INTRODUCTION  TO  CYTOLOGY 

In  the  oogonium  and  segmenting  oospore  of  Facus  Farmer  and 
Williams  (1896,  1898)  described  two  centrospheres  containing  granules 
and  arising  independently  at  opposite  sides  of  the  nucleus.  Strasburger 
(1897)  reported  definite  centrosomes  with  asters  all  through  mitosis. 
In  the  sporeling  he  observed  appearances  indicating  the  division  of  the 
centrosome,  and  concluded  that  the  latter  represents  a  permanent  cell 
organ.  In  a  very  detailed  investigation  Yamanouchi  (1909)  demon- 
strated in  the  antheridium  and  oogonium  two  very  definite  centrosomes, 
which  appear  independently  of  each  other,  become  surrounded  by  con- 
spicuous asters,  and  occupy  the  spindle  poles  during  mitosis  (Fig.  61,  C). 
He  further  showed  that  when  the  sperm  reaches  the  egg  nucleus  a  new 
centrosome  appears  on  the  nuclear  membrane  at  the  point  where  the 
sperm  enters. 

In  Dictyota  dichotoma  Mottier  (1898,  1900)  states  that  in  the  two 
divisions  in  the  tetrasporocyte,  in  at  least  the  first  three  or  four  cell 
generations  of  the  sporeling,  and  in  all  the  vegetative  cells  of  the  tetra- 
sporic  plant  curved  rod-shaped  centrosomes  with  asters  occur  at  the 
spindle  poles,  the  two  having  arisen  by  the  division  of  one  during  the 
early  phases  of  mitosis  (Fig.  21,  D,  E).  Williams  (1904)  further  reports 
that  the  entrance  of  the  sperm  causes  a  centrosome  to  appear  in  the  egg 
cytoplasm.     Centrosomes  in  Nemation  were  described  by  Wolfe  (1904). 

In  Polysiphonia  violacea  (Yamanouchi  1906)  there  are  present  during 
the  prophases  of  every  mitosis  two  centrosome-like  bodies  in  the  kino- 
plasm  at  opposite  sides  of  the  nucleus.  A  little  later  the  small  bodies 
disappear,  while  the  kinoplasm  takes  the  form  of  two  large  centrosphere- 
like  structures  (Fig.  21,  C);  during  the  later  stages  of  mitosis  these  fade 
from  view.  Yamanouchi  believes  that  these  structures  do  not  represent 
permanent  cell  organs,  but  are  formed  de  novo  at  the  beginning  of  each 
mitosis.  Somewhat  similar  temporary  centrospheres,  with  radiations 
but  no  centrosomes,  are  present  in  the  tetrasporocyte  of  Corallina  (Davis 
1898;  Yamanouchi). 

Fungi. — Among  the  fungi  the  best  known  centrosomes  are  those  of  the 
Ascomycetes  (Fig.  22).  Harper  (1895,  1897,  1899,  1905)  described  granu- 
lar disc-shaped  centrospheres  surrounded  by  asters  at  the  poles  of  the 
spindle  in  the  asci  of  Peziza,  Ascobolus,  Erysiphe,  Lachnea,  Phyllactinia, 
and  other  genera.  He  regarded  them  as  permanent  organs  of  the  cell. 
In  a  recent  paper  (1919)  he  speaks  of  the  ascomycete  centrosome  as  a 
structure  differentiated  "as  a  region  of  connection  between  nucleus  and 
cytoplasm  and  for  the  formation  of  fibrillar  kinoplasm."  Harper  be- 
lieved the  ascospore  walls  to  be  formed  by  the  lateral  fusion  of  the  curved 
astral  rays  focussing  upon  the  centrosome,  a  point  disputed  by  Faull 
(1905)  and  others.  Centrosomes  in  additional  genera  were  figured  by 
Guilliermond  (1904,  1905).  In  Gallactinia  succosa  (Marie  1905;  Guillier- 
mond  1911)  a  single  centrosome,  which  arises  within  the  nucleus  with  a 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST 


M 


cone  of  fibrils  extending  toward  the  chromatin,  divides  into  two  which 
take  up  positions  opposite  each  other  at  the  nuclear  membrane,  at  which 
time  asters  develop  in  the  cytoplasm.  Faull  ( 190"))  found  centrosomes  in 
Hydnobolites,  Neoliella,  and  Sordaria;  in  the  last  named  genus  they 
appear  to  be  discoid  while  the  cell  is  in  the  resting  condition  but  round 
and  smaller  during  mitosis.  In  Humaria  rutilans  Mi--  Fraser  (1908) 
observed  two  centrosomes  lying  near  each  other,  each  at  the  apex  of  a 
cone  of  fibers  and  surrounded  by  a  faint  aster.     These  move  apart  and 


If    .'  t~  ■-■■  ■'.  5**-  *•>? 


fitts&F1** 


vj',  «r&M 

V  ,  -  '■      I  ■    %    V*     I   ;  / 


B 


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/?< 


^y. 


\ 


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■/•* 


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n 


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Fig.  22. — Centrosomes  in  ascomycetes. 

A-C,  Phyllactinia  corylea:  division  of  nucleus  in  ascus,  showing  behavior  of  centro- 
somes.    D,   Erisiphe  cichoracearum:  formation  of  ascopore   wall.      {After   Harper,    1905. 

establish  the  spindle  in  the  usual  manner.     Centrosomes  are  also  figured 
in  Ascobolus  and  Lachnea  by  Fraser  and  Brooks  (1909);  in  Otidea  and 
Peziza  by  Fraser  and  Welsford  (1908);  in  Microsphcera  by  Sands  (1907 
and  in  Pyronema  by  Claussen  (1912). 

In  the  Basidiomycete  Boletus  (Levinc  1913)  the  centrosomes  present 
during  the  last  mitosis  in  the  basidium  attach  themselves  to  the  basidium 
wall,  and  in  close  connection  with  them  the  daughter  nuclei  are  recon- 
structed. They  mark  the  points  of  origin  of  the  sterigmata  and  eventu- 
ally pass  into  the  spores. 

6 


82 


ixtkodcctiox  to  cytology 


Bryophytes.-  -The  first  centrosome  known  in  the  liverworts  was  that 
of  Marchantia  described  by  Schottlander  (1893),  according  to  whom  the 
centrosome  in  the  spermatogenous  cells  divides  during  the  anaphases  of 
mitosis,  so  that  each  daughter  nucleus  is  accompanied  by  two  (Fig.  27). 
In  the  gametophytic  colls  certain  minute  bodies  with  radiations  at  the 
poles  of  the  elongated  nucleus  and  of  the  spindle  were  believed  by  Van 
Hook  (1900)  to  represent  centrosomes.  Centrospheres  with  conspicuous 
radiations  but  without  true  centrosomes  were  described  in  the  mitoses  of 
the  germinating  spore  of  Pellia  by  Farmer  and  Reeves  (1894),  Davis 
(1901),  and  Chamberlain  (1903).  Gregoire  and  Berghs  (1904),  however, 
pointed  out  that  the  centrospheres  observed  by  the  foregoing  writers  in 
Pellia  are  in  reality  only  appearances  due  to  the  intersection  of  numerous 
astral  rays,  and  are  not  distinct  bodies. 


*        1  '    '     "'    : '     -  s» 


Fig.  23. — Centrosomes  in  Preissia  quadrata. 

A,  in  fertilized  egg  just  prior  to  nuclear  fusion,     B,  in  cells  of  young  embryo.      (After 
Graham,  1918.) 


In  the  cells  of  Preissia  quadrata  Miss  Graham  (1918)  has  more  recently 
made  some  observations  of  much  interest.  She  describes  and  figures  two 
distinct  centrosomes  with  a  few  astral  rays  in  the  cytoplasm  of  the  fer- 
tilized egg,  at  the  time  when  the  sexual  nuclei  are  approaching  each  other 
and  in  contact  (Fig.  23,  A).  This,  together  with  Yamanouchi's  observa- 
tion on  Fucus  and  that  of  Williams  on  Dictyota,  cited  above,  suggests 
that  in  certain  plants,  as  in  animals,  the  formation  of  centrosomes  and 
asters  in  the  egg  cytoplasm  is  in  some  way  induced  by  the  entrance  of  the 
sperm.  Similar  appearances  have  been  noted  by  Meyer  (1911)  in  Cor- 
sinia  and  by  Florin  (1918)  in  Riccardia  (Aneura).  Centrosomes  were 
also  observed  by  Miss  Graham  in  the  four-celled  embryo  of  Preissia 
(Fig.  23,  B),  this  being  one  of  the  only  cases  in  which  centrosomes  have 
been  seen  in  non-spermatogenous  cells  in  plants  above  the  algae. 

Conclusion. — With  regard  to  centrosomes  in  plants,  it  may  be  con- 
cluded from  the  above  review  that  although  there  is  no  adequate  evidence 
for  their  existence  in  the  cells  of  angiosperms,  they  are  clearly  present  in 
many  algae,  fungi,  and  probably  certain  bryophytes.  where  they  perform 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST  83 

definite  functions  in  the  life  of  the  cell.  The  question  of  centrosomes  in 
the  spermatogenous  cells  of  bryophytes,  pteridophytes,  and  gymnosperms 
is  dealt  with  in  the  following  discussion  of  the  blepharoplast. 

THE  BLEPHAROPLAST 

Occurrence.  -The  blepharoplast,  as  indicated  by  the  name  given  to 
it  by  Webber  (1897),  is  the  cilia-bearing  organ  of  the  cell.  Blepharo- 
plasts  of  one  kind  or  another  are  found  generally  in  the  motile  cells  of 
plants  and  animals,  such  as  motile  unicellular  organisms  (Flagellata 
Ciliata,  etc.),  swarm  spores,  spermatozoa,  and  spermatozoids;  and  also 
in  cells  which,  though  not  freely  motile  themselves,  have  motile  organs 
performing  other  functions,  as  in  the  case  of  ciliated  epithelium.  In 
plants  blepharoplasts  are  most  conspicuously  displayed  in  the  sperma- 
togenous cells  of  bryophytes,  pteridophytes,  and  gymnosperms  cycads 
and  Ginkgo),  where  their  striking  resemblance  to  ordinary  centrosomes 
has  led  to  much  controversy  over  their  nature.  Some  cytologists  have 
regarded  the  blepharoplast  as  a  more  or  less  modified  centrosome,  where- 
as others  have  contended  that  it  is  a  special  kinoplasmic  or  cytoplasmic 
organ  distinct  from  the  centrosome.  In  recent  years  the  evidence  has 
tended  strongly  to  support  the  former  view. 

In  the  following  pages  the  blepharoplasts  of  various  organisms  and 
the  manner  in  which  they  function  in  the  development  of  the  motor 
apparatus  will  be  described  in  some  detail.  Attention  will  be  given 
chiefly  to  the  situations  found  in  plants;  the  corresponding  phenomena 
in  animals  will  be  more  briefly  considered. 

Flagellates.— In  the  flagellates  several  types  of  flagellar  apparatus 
are  found  (see  Minchin  1912,  pp.  82  ff.,  262-3):  in  one  series  of  forms 
the  cell  contains  a  single  nucleus  and  "centriole,"  the  latter  functioning 
both  as  a  centrosome  and  as  a  blepharoplast.     The1  centriole  may  lie 
either  against  or  within  the  nucleus,  so  that  the  flagellum  which  grows 
from  it  appears  to  arise  directly  from  the  nucleus  (Mastigina) ;  in  other 
forms    (Mastigella)    the  centriole  is  quite  independent   of  the  aucleus. 
In  a  second  series  of  forms  a  single  nucleus  and  centrosome  are  present 
and  in  addition  one  or  more  blepharoplasts.     Three  conditions  have  been 
distinguished  here:  (a)  at  the  time  of  cell-division   the  blepharoplasts 
and  flagella  are  lost,  new  blepharoplasts  arising  from  the  centrosomes 
during  or  after  mitosis;  (b)  the  blepharoplasts  may  persist,  dividing  to 
form  daughter  blepharoplasts  from  which  new  flagella  arise  {PolytomeUa 
(e)  the  centrosome  divides  to  furnish  a  blepharoplast  which  subdivides  to 
two:  a  distal  blepharoplast   or  basal  granule  of  the   flagellum,  and  a 
proximal  blepharoplast   or  "anchoring  granule'    al    the  surface  of  the 
nucleus,  the  two  being  connected  by  a   delicate  strand   known  as   the 
rh/zoplast,  rhizonema,  or  centrodesmose  {Peranema  trichophorum        Entz 
(1918)  has  recently  reinvestigated  the  structure  of  Polytoma  uvella,  firsl 


84 


INTRODUCTION  TO  CYTOLOGY 


described  by  Dangeard   (1901),   and  finds  the  elaborate  organization 

shown  in  Fig.  24. 

In  a  third  series  of  forms  two  nuclei  are  present:  a  principal  or  trophic 

nucleus  and  an  accessory  or  kinetic 
nucleus.  Here  there  are  apparently 
three  conditions :  (a)  a  single  centrosome, 
associated  with  the  kinetonucleus,  acts 
both  as  a  blepharoplast  and  as  a  division 
center;  (6)  usually  both  nuclei  have 
centrosomes  associated  with  them,  the 


• 


Fig.  24.  Fig.  25. 

Fig.  24. — Diagram  of  structure  of  Polytoma  uvella.      (After  Entz,  1918.) 
a,  end-piece  of  flagellum.     b,  uniform  portion  of  flagellum.     c,  lateronema.     d,  baso- 
plast  or  basal  granule,     e,  contractile  vacuole.     /,  cell  envelope,     g,  eyespot.     h,  rhizonema. 
i,  karyoplast  or  anchoring  granule,     j,  centronema.     k,  nucleolus.     I,  nuclear  membrane, 
m,  starch,     n,  surface  of  protoplast. 

Fig.  25. — Trypanosoma  theileri. 

A,  flagellum  inserted  on  basal  granule.     B,  formation  of  new  flagellum  from  daughter 
basal  granule  after  division;  nucleus  dividing.      (After  Hartmann  and  Noller,  1918.) 

one  lying  near  or  within  the  kinetonucleus  acting  as  the  blepharoplast; 
(c)  it  is  possible  that  in  some  cases  there  is  a  blepharoplast  distinct  from 
the  centrosomes  accompanying  the  two  nuclei. 

In  the  trypanosomes  (Fig.  25)  the  recent  researches  of  Kuczynski 
(1917)  and  Hartmann  and  Noller  (1918)  have  shown  that  the  flagellum 


THE  CENTROSOME  AM)  THE  BLEPHAROPLAST 


v 


is  inserted  on  a  "basal  granule"  (centre-some?)  very  near  the  "blepharo- 
plast"  (kinetonucleus?).  At  the  time  of  cell-division  the  trophic  nucleus, 
blepharoplast,  and  basal  granule  all  divide,  the  division  of  the  blepharo- 
plast  showing  certain  features  suggesting  mitosis.  Although  earliei 
investigators  thought  the  flagellum  also  split,  the  above  named  workers 
find  that  the  old  flagellum  remains  attached  to  one  of  the  daughter  basal 
granules  while  a  new  flagellum  grows  out  from  the  other  daughter  granule. 

In  flagellate  organisms,  therefore,  the  centrosome  and  the  blepharo- 
plast  clearly  stand  in  very  intimate  relationship  with  one  another:  in 
some  of  the  forms  they  are  one  and  the  same  organ. 

Thallophytes. — Among  the  earliest  investigations  of  the  blepharoplast 
in  algae  were  those  of  Strasburger  (1892,  1900).  During  the  development 
of  the  zoospores  of  (Edogonium,  Cladophora,  and  Vaucheria  Strasburger 


aim 


r  '<£>  ~  ■■'-  <■>-  vx  -■ 


, 


/ 


*WK3 


\ 


HI 


Fig.  26.— Blepharoplasts  in  Thallophytes. 

A-D,  formation  of  the  cilia-bearing  ring  in  the  zoospore  of  Derbe&ia.      {After  Davis, 
1908.)      E,  Stemonitis  flaccida:  cilia  growing  from  centrosomes  during  late  stage  <>f  ili\  i-; 
in  the  formation  of  swarmers.      (After  Jahn,  1904.) 


isior 


found  that  the  nucleus  approaches  the  plasma  membrane,  which  at  that 
point  forms  a  lens-shaped  thickening.  From  this  structure  grow  out  th< 
cilia,  and  at  the  base  of  each  a  small  refractive  granule  is  present.  Tin 
blepharoplasts  of  the  higher  groups  were  believed  by  Strasburger  to  hav< 
been  derived  from  such  swollen  ectoplasmic  organs  of  the  alg«,  and  that 
all  of  them  are  morphologically  distinct  from  centrosomes.  Dangeard 
(1898)  likewise  found  a  deeply  staining  granule  at  the  base  of  the  cilia 
in  Chlorogonium. 

In  Hydrodictyon  (Timberlake  1902)  the  cilia  are  inserted  on  a  smal] 
body  lying  in  contact  with  the  plasma  membrane  and  joined  with  th< 
nucleus  by  a  delicate  protoplasmic  strand.     The   possible   relationship 
of  this  body  with  the  granules  seen  occupying  the  spindle  poles  during 
the  formation  of  the  spore  cells  was  not  determined.     In  the  young 


86 


INTRODUCTION  TO  CYTOLOGY 


zoospore  cell  of  Derbesia  (Davis  1908)  the  nucleus  migrates  toward  the 
plasma  membrane,  and  from  it  many  granules,  which  are  not  centrosomes, 
move  out  along  radiating  strands  of  cytoplasm  to  the  surface  of  the  cell, 
where  by  fusion  they  form  a  ring-shaped  structure  from  which  the  cilia 
develop  (Fig.  26,  A-D).  In  the  developing  spermatozoid  of  Chara 
(Belajeff  1894;  Mottier  1904)  the  blepharoplast  arises  as  a  differentiation 
of  the  plasma  membrane  and  bears  two  cilia.  No  centrosomes  or  other 
granules  were  seen  at  the  base  of  the  cilia,  although  Schottlander  (1893) 
had  previously  reported  centrosomes  in  the  cells  of  the  spermatogenous 
filament. 

In  the  zoospore  of  the  fungus  Rhodochytrium  (Griggs  1904)  there  is  a 
deeply  staining  body  at  the  insertion  point  of  the  cilia;  this  is  connected 
by  fine  cytoplasmic  fibers  with  the  nucleus.  In  the  myxomycete  Stemo- 
nitis  Jahn  (1904)  made  an  observation  that  is  highly  suggestive  in  con- 
nection with  the  question  of  the  relationship  of  the  centrosome  and  the 
blepharoplast.  At  the  last  mitosis  in  the  formation  of  the  swarmers  the 
spindle  poles  are  occupied  by  centrosomes,  and  during  the  anaphases 
the  flagella  of  the  resulting  swarmers  grow  out  directly  from  these  cen- 
trosomes (Fig.  26,  E),  just  as  in  the  spermatocytes  of  certain  insects 
(p.  95). 


W^m 


'.L-l-^J 


Fig.  27. — Spermatogenesis  in  Marchantia. 

b,    blepharoplast;    c,    centrosome;    c.n.,    " chromatoider    Nebenkorper ; "    n,    nucleus. 

I  After  Ikeno,  1903.) 

Bryophytes. — Among  the  bryophytes  the  blepharoplasts  of  Mar- 
chantia and  Fegatella  (Conocephalus)  have  received  much  attention. 
According  to  Ikeno  (1903)  a  centrosome  comes  out  of  the  nucleus  at 
each  spermatogenous  division  in  Marchantia  and  divides  to  form  two 
which  separate  to  opposite  sides  of  the  cell,  occupy  the  spindle  poles, 
and  disappear  at  the  close  of  mitosis:  it  is  possible  that  they  are  included 
in  the  daughter  nuclei.  After  the  last  (diagonal)  division,  however,  they 
remain  in  the  cytoplasm  as  the  blepharoplasts,  elongating  and  bearing- 
two  cilia   (Fig.   27).     Another  body,   the  chromatoider  Nebenkorper,   is 


77/ A'  CENTROSOME  AND  THE  BLEPHAROPLAST 


also  present  in  the  cytoplasm.  Similar  in  mosl  points  is  the  account 
of  Schaffner  (1908).  Miyake  (1905),  as  the  resull  of  his  studies  on 
Marchantia,  Fegatella,  Pellia,  Aneura,  and  Makinoa,  believes  thai  such 
liverwort  centrosomes  arc  merely  centers  of  cytoplasmic  radiation,  and 
inclines  toward  the  view  of  Strasburger  that  the  blepharoplasl  and  the 
centrosome  are  not  homologous  structures.  Escoyez  (1(.)()7)  finds  two 
" corpuscles "  appearing  in  contact  with  the  plasma  membrane  in  each 
cell  of  the  penultimate  generation  in  the  antheridia  of  Marchantia  and 
Fegatella;  they  occupy  the  spindle  poles  and  function  as  blepharoplasts 
in  the  spermatids  (the  cells  which  transform  directly  into  spermatozoids 
Bolleter  (1905)  believes  the  centrosome-like  body  in  Fegatella  to  arise 
within  the  nucleus. 

In  the  antheridium  of  Riccia  Lewis  (1906)  reported  centrosome-like 
bodies  in  both  the  early  and  diagonal  divisions.     They  apparently  an 
de  novo  in  the  cytoplasm  prior  to  each  mitosis,  showing  no  continuity 
through  succeeding  cell  generations  except  after  the  last  mitosis,  when 
they  persist  and  become  the  blepharoplasts. 


-  3U 


!. 


*      '-'••/ 


■ 
i  '. 


xJHf\ 


• 


1   ■y'7^>   ': 


':  ■  • 


-'•■' 


\. 


fete 

'v.; 


) 


9©KJ 
■  -•    i  - 


Fig.   l's.     Spermatogenesis  in  Blasia. 
b,  blepharoplast;  rc,  nucleus.      X  4200.     (.!//</■  .n7/#i/-/».  L920 

Woodburn  (1911,  1913,  1915)  has  given  accounts  of  spermatogenesis 
in  Porella,  Asterella,  Marchantia,  Fegatella,  Blasia,  and  Mnium.  He 
finds  that  the  blepharoplast  is  first  distinguishable  as  a  special  granule  in 
the  cytoplasm  of  the  spermatid  and  holds  that  it  represents,  as  Mottier 
(1904)  had  formerly  suggested,  an  individualized  pott  ion  of  the  kinoplasm 
arising  de  novo  in  certain  spermatogenous  cells.  In  a  more  recent  con- 
tribution (Sharp  1920)  it  has  been  shown  that  in  BUma  (Fig.  28)  a 
blepharoplast  is  present  at  each  spindle  pole  during  all  stages  of  the  last 
spermatogenous  mitosis,  and  that    in   the  spermatid  it    fragments  as  it 


48  INTRODUCTION  TO  CYTOLOGY 

becomes  transformed  into  the  cilia-bearing  thread  after  tlie  manner  of 
the  blepharoplast  of  Equisetum,  described  below. 

Spermatogenesis  in  Pellia,  Atrichum,  and  Mnium  has  been  described 
by  M.  Wilson  (1911).  In  Mnium  and  Atrichum  the  spermatogenous 
divisions  show  no  centrosomes,  whereas  in  Pellia  centrospheres,  and 
probably  centrosomes,  are  present  during  the  later  mitoses.  The  origin 
of  the  blepharoplast  as  here  described  is  very  peculiar.  In  the  spermatid 
of  Mnium  a  number  of  bodies  are  said  to  separate  from  the  nucleolus  and 
pass  out  into  the  cytoplasm  where  they  coalesce  to  form  a  limosphere. 
The  nucleolus  then  divides  into  two  masses,  both  of  which  pass  into  the 
cytoplasm;  one  functions  as  the  blepharoplast  and  the  other  gives  rise 
to  an  accessory  body.  In  Atrichum  the  first  body  separated  from  the 
nucleolus  becomes  the  blepharoplast,  a  second  forms  the  limosphere,  and 
a  third  the  accessory  body.  In  all  three  plants  the  blepharoplast  goes 
to  the  periphery  of  the  cell  and  grows  out  into  a  thread-like  structure 
along  the  plasma  membrane.  The  nucleus  then  moves  against  this 
thread  and  the  two  grow  together  to  form  the  spirally  coiled  spermatozoid. 
Two  cilia  grow  out  from  the  anterior  end  of  the  blepharoplast. 

The  most  detailed  and  critical  of  all  researches  on  the  motile  cells 
of  bryophytes  are  those  of  C.  E.  Allen  (1912,  1917)  on  Polytrichum 
(Fig.  29).  The  first  of  these  papers  contains  a  description  of  the  cyto- 
logical  phenomena  accompanying  the  multiplication  of  the  spermato- 
genous cells  (androgones)  up  through  the  last  mitosis,  which  differentiates 
the  spermatids  (androcytes) .  In  the  cytoplasm  of  all  the  androgones 
there  is  a  deeply  staining  kinoplasmic  mass;  in  the  early  androgones  this 
has  the  form  of  a  flat  plate,  while  in  the  later  ones  it  consists  of  a  group 
of  granules  (kinetosomes).  Prior  to  each  mitosis  the  plate  or  group 
divides  to  daughter  plates  or  groups  which  pass  to  the  daugher  cells. 
In  the  cells  of  the  penultimate  generation  (androcyte  mother-cells)  there 
are  no  kinetosomes,  but  instead  a  spherical  "central  body"  with  radia- 
tions. This  body  divides  into  two  which  move  apart  and  occupy  the 
spindle  poles  during  the  last  mitosis.  Each  resulting  androcyte  therefore 
has  one  such  body,  which  functions  as  the  blepharoplast.  Allen  does  not 
regard  the  kinetosomes  as  definite  morphological  entities,  but  rather  as 
masses  of  reserve  kinoplasm.  The  blepharoplast,  however,  is  a  definite 
cell  organ,  and  although  Allen  inclines  toward  the  view  that  it  is  the 
homologue  of  a  centrosome  he  regards  the  question  as  an  open  one. 
Sapehin  (1913)  looks  upon  these  bodies  as  plastids. 

Allen's  second  paper  deals  with  the  transformation  of  the  androcyte 
(spermatid)  into  the  spermatozoid.  The  blepharoplast  elongates  to 
form  a  uniform  rod  and  develops  two  cilia  from  near  its  anterior  end. 
The  nucleus  moves  against  the  middle  portion  of  the  blepharoplast  and 
the  two  elongate  together  in  close  union  to  form  the  body  of  the  sperma- 
tozoid,  the  blepharoplast   projecting  beyond  the  anterior  end  of  the 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST 


gg 


nucleus.  At  aboul  the  time  the  blepharoplasl  begins  to  elongate  a 
limosphere  appears  in  the  cytoplasm  and  lakes  up  a  position  near  the 
anterior  end  of  the  blepharoplast.  Here  it  divides,  giving  rise  to  a  small 
apical  body  that  remains  visible  at  the  end  of  the  blepharoplasl  until  a 
comparatively  late  stage.  The  remaining  portion  of  the  Limosphere  may 
be  seen  lying  against  the  nucleus  until  the  maturity  of  the  spermatozoid 


B 


1  v-  *^-4#*f  M-h  n 


/■?  ■  / 


V 


G 


s 


t 


s 


, 


■ 


t 


i 


Fig.  29. — Spermatogenesis  in  Polytrichum. 
A-D,  androgones,  showing  behavior  of  kinoplasmic  plates  and  kinetosomes  (*)  during 
lmitoss.  E-G,  androcyte  mother-cells,  showing  division  of  central  body.  //.  telophase 
of  last  mitosis;  each  androcyte  has  a  blepharoplasl  (b).  I-K,  stages  in  the  transformation 
of  the  androcyte  into  the  spermatozoid:  a,  apical  body;  /.  limosphere;  n,  nucleus;  />. 
percnosome.     /,,  mature  spermatozoid.      X  2535.     (After  Allen,   L912,    L917 

Another  body,  the  percnosome,  is  also  seen  in  the  cytoplasm  a1  certain 
stages.  In  the  opinion  of  Allen  the  limosphere  is  probably  identical  with 
the  chromatoider  Nebenlnrjur  described  by  [keno  in  Marchantia,  and  the 
percnosome  with  what  M.  Wilson  (1911)  terms  the  accessory  body.  The 
apical  body  is  here  described  for  the  first  time  by  Allen. 

Pteridophytes.- -The  early  papers  dealing  with  the  spermatozoid  of 
pteridophytes,  such  as  those  of  Buchtien  (1887),  Campbell  (1887),  Bela- 


90 


INTRODUCTION  TO  CYTOLOGY 


jeff  (1888),  Guignard  (1889),  and  Schottlander  (1893),  give  but  little 
information  concerning  \\\r  developmenl  of  the  blepharoplast.  Our  more 
definite  knowledge  of  this  subject  dates  from  1897,  when  Bela  jeff  published 
three  short  papers.  In  the  first  of  these  (1897a)  it  was  stated  that  the 
fern  spermatozoid  consists  of  a  thread-shaped  nucleus  and  a  plasma  band. 
with  a  great  many  cilia  growing  out  from  the  latter.  In  the  plasma  band 
is  enclosed  a  thin  thread  which  arises  by  the  elongation  of  a  small  body 
seen  in  the  spermatid.  In  the  second  paper  (18976)  the  blepharoplast  of 
Equisi  turn  was  first  described  as  a  crescent-shaped  body  lying  against  the 
nucleus  of  the  spermatid;  this  body  stretches  out  to  form  the  cilia-bearing 
thread.  The  third  contribution  (1897c)  is  a  short  account  of  the  trans- 
formation of  the  spermatid  into  the  spermatozoid  in  Cham,  Equisetum, 


Fig.  30. — Spermatogenesis  in  Equisetum  arvense,  showing  behavior  of  blepharoplast 
(centrosome)  in  last  spermatogenous  mitosis  and  in  transformation  of  spermatid  into  sper- 
matozoid.     X  1900.     (After  Sharp,  1912.) 

and  ferns.  In  all  these  forms  a  small  body  elongates  to  form  a  thread 
upon  which  small  swellings  arise  and  grow  out  into  cilia.  In  a  comparison 
with  animal  spermatogenesis  Belajeff  here  homologized  the  blepharoplast, 
the  thread  to  which  it  elongates,  and  the  cilia  of  the  plant,  with  the 
centrosome,  middle  piece,  and  tail  (perhaps  only  the  axial  filament), 
respectively,  of  the  animal.  In  the  following  year  (1898)  he  figured  the 
details  made  out  in  Gymnogramme  and  Equisetum.  In  Gymnogramme  the 
two  blepharoplasts  appear  at  opposite  sides  of  the  nucleus  in  the  spermatid 
mother-cell,  whereas  in  Equisetum  a  single  blepharoplast  is  first  figured 
lying  close  to  the  nucleus  of  the  spermatid.  More  recently  it  has  been 
shown  (Sharp  1912)  that  the  blepharoplast  of  Equisetum  (Fig.  30)  appears 
first  in  the  cells  of  the  penultimate  generation;  there  it  divides  to  two 
which  separate  and  establish  between  them  the  achromatic  figure  after 
the  manner  of  animal  centrosomes.     At  the  close  of  mitosis  the  blepharo- 


THE  CENTROSOME  AM)  THE  BLEPH  VROPL  1ST 


91 


plasl  in  each  spermatid  fragments  into  a  number  of  pieces;  these  later 
join  to  form  a  continuous  beaded  thread  from  which  the  cilia  grow  out. 
In  Equisetum  the  elongating  nucleus  and  blepharoplasl  do  no1   become 

closely  joined,  but  arc  held  together  only  by  the  rather  abundant  cyto- 
plasm. The  spermatozoid  is  multiciliate  like  that  of  all  other  pterido- 
phytes  with  the  exception  of  Lycopodium,  Phylloglossum,  and  Selaginella: 
in  these  three  genera  the  spermatozoids  are  biciliate  like  those  of  the 
bryophytes. 

The  most  careful  work  on  the  blepharoplasl  of  homosporous  ferns  is 
that  of  Yamanouchi  (1908)  on  Nephrodium  (Fig.  31).  In  this  form  no 
centrosomes  are  found.  The  two  blepharoplasts, 
which  arise  de  novo  in  the  cytoplasm  of  the  sperm- 
atid mother-cell,  take  no  active  part  in  nuclear 
division,  merely  lying  near  the  poles  of  the  spindle. 
In  the  spermatid  the  blepharoplast  elongates  spirally 
in  close  union  with  the  nucleus  to  form  the  body  of 
the  spermatozoid.  In  Adiantum  and  Aspidium  Miss 
R.  F.  Allen  (1911)  and  Thorn  (1899)  see  the  blepharo- 
plast first  in  the  spermatid. 

One  of  the  most  interesting  blepharoplasts  is  that 
of  Marsilia  (Fig.  32),  first  described  by  Shaw  (1898). 
According  to  Shaw  a  small  granule,  or  "blepharo- 
plastoid,"  appears  near  each  daughter  nucleus  of  the 
mitosis  which  differentiates  the  grandmother-cell  of 
the  spermatid  (the  second  of  the  four  spermatogenous 
mitoses).  During  the  next  (third)  division  the 
blepharoplastoid  divides  but  soon  disappears,  and  a 
blepharoplast  appears  near  each  spindle  pole.  In  the 
next  cell  generation  (spermatid  mother-cell)  the  in  Lasii  spermatogen- 
blepharoplast  divides  into  two  which  are  situated  at  g^ito08fi8'biePw 
the  spindle  poles  during  the  final  mitosis.  In  the 
spermatid  the  blepharoplast  gives  rise  to  several 
granules  by  a  sort  of  fragmentation;  these  together 
form  a  thread  which  elongates  spirally  in  close  union 
with  the  nucleus  and  bears  many  cilia.  The  spermatozoid  isof  the  usual 
fern  type,  with  several  coils  and  a  cytoplasmic  vesicle.  Shaw  saw  in  the 
foregoing  facts  no  ground  for  the  homology  of  the  blepharoplasl  and  the 
centrosome.  Belajeff  (1899)  found  thai  centrosomes  occur  al  the  Bpindle 
poles  during  all,  excepting  possibly  the  first,  of  the  tour  divisions  which 
.result  in  the  16  spermatids.  He  reported  thai  after  each  mitosis  the 
centrosome  divides  into  two  which  occupy  the  spindle  poles  during  the 
succeeding  mitosis,  and  in  the  spermatids  perform  the  usual  functions  oi 
blepharoplasts.  Belajeff  regarded  this  as  a  strong  confirmation  of  his 
theory  that  the  blepharoplast  and  centrosome  are  homologous  organs. 


Fig.  .'i  1  .  T  w  o 
stages  in  the  sperma- 
togenesis of   A  •  /■' 

ilium. 

A.    blepharoplasts 

iicai'  poles  of  spindle 


plasl     Dear     nucleus. 
Nebenkern    at     left. 
i  A/ti  r      Yamanott 
1908 


92 


INTRODUCTION  TO  CYTOLOGY 


The  results  of  Shaw  were  in  the  main  confirmed  by  the  later  work  of 
Sharp  (1914),  who,  however,  saw  in  the  achromatic  structures  accom- 
panying the  blepharoplast  striking  evidence  in  favor  of  Belajeff's  view 
of  its  homology.  In  line  with  this  conclusion  the  suggestion  has  recently 
been  made  (Sharp  1920)  that  the  fragmentation  of  the  blepharoplast 
in  Bla.sia,  Equisetum,  Marsilia,  and  the  cycads  may  be  homologized  with 
the  normal  division  exhibited  by  ordinary  centrosomes. 


Fig.  32. — Spermatogenesis  in  Marsilia  quadrifolia. 
A,  first  spermatogenesis  mitosis;  no  centrosomes.  B,  second  mitosis,  centrosomes 
present.  C,  third  mitosis;  centrosomes  present;  old  centrosome  divided  and  degenerating 
in  cytoplasm.  D,  penultimate  spermatogenous  cell;  daughter  centrosomes  separating. 
E,  last  spermatogenous  mitosis;  blepharoplasts  (centrosomes)  becoming  vacuolate.  F,  frag- 
mentation of  blepharoplast  in  spermatid.  G,  transformation  of  spermatid  into  spermato- 
zoid.     H,  free  swimming  spermatozoid.      X  1400.      {After  Sharp,  1914.) 

Gymnosperms. — The  first  known  blepharoplast  in  plants  above  the 
algae  was  discovered  in  Ginkgo  by  Hirase  in  1894.  He  observed  two,  one 
on  either  side  of  the  body  cell  nucleus,  and  because  of  their  great  simi- 
larity to  certain  structures  in  animal  cells  he  believed  them  to  be  attrac- 
tion spheres.  In  1897  Webber  observed  the  same  bodies,  and  noted  their 
cytoplasmic  origin.  On  account  of  certain  differences  between  these 
organs  and  ordinary  centrosomes  he  expressed  the  opinion  that  they  are 
not  true  centrosomes,  but  distinct  organs  of  spermatogenous  cells. 
The  blepharoplast  of  Ginkgo  was  later  investigated  by  Fujii  (1898,  1899, 
1900)  and  Miyake  (1906). 


THE  CENTROSOVE  AXD  THE  BLEPHAROPLAST 


93 


In  1897  and  1901  Webber  described  the  blepharoplasl  of  Zamia  '  1  ig. 
33).  Up  to  the  time  of  the  division  of  the  body  cell  the  two  blepharo- 
plasts,  which  arise  de  novo  in  the  cytoplasm,  are  surrounded  by  radiations, 
but  they  have  no  part  in  the  formation  of  the  spindle,  which  is  entirely 
intranuclear.  During  mitosis  they  lie  opposite  the  poles,  increase  greatly 
in  size,  become  vacuolate,  and  break  up  to  many  granules:  these  in  the 
spermatid  coalesce  to  form  a  spirally  coiled  cilia-bearign  band  lying  just 
inside  the  cell  membrane.  In  his  full  account  (1901)  Webber  gives  an 
extensive  discussion  of  the  homology  of  the  blepharoplast. 


Ml 


w 

A. 

- 

B 


Wfmi, 

- 
c 


■ 


/ 


.     i  • 


Fig.  33. — Spermatogenesis  in  Zamia. 
A-E,  five  stages  in  the  vacuolation  and  fragmentation  of  the  blepharoplasl  during  the 
mitosis  differentiating  the  spermatids.     F,  the  two  spermatozoida  in  tht>  end  of  the  pollen 
tube;  prothallial  and  stalk  cells  below.     Compare    Fig.   34.     A-D,    X   350;    E,     •    L200. 
{After  Webber,  1901.) 


In  Ikeno's  (1898)  account  of  gametogenesis  and  fertilization  in  Cycas 
it  was  shown  that  the  blepharoplast s  appear  in  the  body  cell,  lie  opposite 
the  spindle  poles  during  mitosis,  and  break  up  to  granules  which  fuse  to 
form  the  spiral  band  in  a  manner  similar  to  that  described  by  Webber  for 
Zamia.  The  behavior  of  the  blepharoplasl  in  Microcycas  (Caldwell 
1907)  is  essentially  the  same. 

Chamberlain  (1909)  observed  in  the  cytoplasm  of  the  body  cell  of 
Dioon  (Fig.  34)  a  number  of  very  minute  "black  granules"  which  he  was 
inclined  to  believe  originate  within  the  nucleus.  Very  soon  two  undoubted 
blepharoplasts  are  present,  and  are  apparently  formed  by  the  enlarge- 


94 


INTRODUCTION  TO  CYTOLOGY 


menl  oi  two  oi  the  black  granules.  Very  conspicuous  radiations  develop 
about  them,  and  after  mitosis  they  form  ribbon-like  cilia-bearing  bands 
in  the  spermatids  as  in  the  other  cycads. 


rif)r\  ■  ...  -\ 


A 


B 


Ctarlei  J .  Otaml*  rUn  del. 


*•-* 


Fig.  34. — Spermatogenesis  in  Dioon  edule. 

diff^&fed  °ex  18q(»h  br°}k  fa^!eS  £  7*°^™'      X  189°-     *  two  blepharoplasts 

below       V  2qV       n    f  +  +  y  Ce  V^  u  tW°  blePhar«P^sts;  prothallial  and  stalk  cells 

'    X  1890       F     T^r  °    blepharoplast  in  spermatid  as  spiral  band  begins  to 

m.       X  1890.      £,  portion  of  edge  of  spermatozoid,  showing  spiral  band  out  at  two  noints 

and  cdia  growing  from  it.      X  945.      {After  Chamberlain.  1909.)  P 

Ikeno  in  1898  expressed  the  opinion  that  the  blepharoplast  of  Ginkgo 
a  ad  the  cycads  is  a  true  centrosome,  a  view  shared  by  Chamberlain  (1898) 
and  Guignard  (1898).  Two  additional  papers  dealing,  with  this  subject 
were  published  by  Ikeno  (1904,  1906).  In  the  firsr of  these  he  made 
comparisons  with  analogous  phenomena  in  animals  which  he  believed  to 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST  95 

sustain  the  homologies  suggested  by  BelajefT.  Be  pointed  oul  thai  in 
Marchantia  centrosomes  are  present  in  all  the  spermatogenoua  divisions, 
whereas  in  other  liverworts  they  appear  much  later,  and  from  this  he 
argued  that  the  bryophytes  show  various  stages  in  the  elimination  of  the 

centrosome.  He  strongly  reasserted  his  belief  thai  blepharoplasts  are 
centrosomes,  and  spoke  of  the  "transformation  <>f  ;i  centrosome  into  a 
blepharoplast"  in  the  development  of  a  spermatid  into  a  spermatozoid. 
The  ectoplasmic  blepharoplasts  of  the  algae  were  also  held  to  be  derived 
from  centrosomes.  In  the  second  paper  he  insisted  less  si  rongly  upon  t  he 
morphological  identity  of  all  blepharoplasts,  separating  them  into  three 
categories:  (1)  centrosomatic  blepharoplasts,  including  those  of  the  myxo- 
mycetes,  bryophytes,  pteridophytes,  and  gymnosperms;  (2)  plasmoder- 
mal  blepharoplasts,  including  those  of  Cham  and  some  Chlorophyceae; 
(3)  nuclear  blepharoplasts,  found  only  in  a  few  flagellate-. 

For  a  further  discussion  of  this  question  the  student  is  referred  to 
the  present  author's  papers  on  Equisetum  and  Marsilia.  The  main 
conclusions  reached  may  be  stated  in  t  wo  extract  s  from  the  former  paper: 

Although  limited  to  a  single  mitosis  in  the  antheridium,  the  blepharoplasl 
fof  Equisetum]  retains  in  its  activities  the  most  unmistakable  evidence-  of  a 
centrosome  nature,  and  at  the  same  time  shows  a  metamorphosis  strikingly  like 
that  in  the  cycads.  In  thus  combining  the  main  characteristics  of  true  centro- 
somes with  the  peculiar  features  of  the  most  advanced  blepharoplasts,  it  reveals 
in  its  ntogen3r  an  outline  of  the  phylogeny  of  the  blepharoplast  as  it  i>  Been 
developing  through  bryophytes,  pteridophytes,  and  gymnosperms,  from  a  func- 
tional centrosome  to  a  highly  differentiated  cilia-bearing  organ  with  very  lew 
centrosome  resemblances. 

The  activities  of  the  blepharoplast  in  Equisetum  [Marsilia,  and  Blasia], 
taken  together  with  the  behavior  of  recognized  true  centrosomes  in  plants  ami 
analogous  }  henomena  in  •animals,  are  believed  to  constitute  conclusive  evidence 
in  favor  of  the  theory  that  the  blepharoplasts  of  bryophytes,  pteridophytes,  and 
gymnosperms  are  derived  ontogenetically  or  phylogenetically  from  centre-.. me-. 

Animals. — The  early  researches  of  Moore  (1895),  Meves  I  L897,  L899  . 
Korff  (1899),  Paulmier  (1899),  and  many  other  more  recent  investigators 
have  established  the  fact  that  the  centrosome  (or  centrosomes)  of  the 
animal  spermatid  plays  an  important  role  in  the  formation  of  the  motor 
apparatus  of  the  spermatozoon,  the  axial  filament  of  the  tail  growing  oul 
directly  from  it  (Fig.  35).  Henneguy  (1898)  even  saw  flagella  attached 
to  the  centrosomes  of  the  mitotic  figure  in  the  spermatocyte  of  an  insect. 
an  observation  which  has  been  often  repeated.  Wilson  L900,  p.  17.'. 
concludes  that  "the  facts  give  the  strongest  ground  for  the  conclusion 
that  the  formation  of  the  spermatozoids  agrees  in  its  essential  featuri 
with  that  of  the  spermatozoa  .  .  .  and  that  the  blepharoplasl  is 
without  doubt  to  be  identified  with  the  centrosome. 


96 


1STR0DVCTI0N  TO  CYTOLOGY 


Although  there  is  comparatively  little  question  that  the  granule  at  the 
base  of  the  flagellum  in  the  flagellates,  like  the  body  from  which  the  axial 
filament  of  the  spermatozoon  grows,  is  of  centrosomic  nature,  the  nature 
of  the  basal  granules  in  Ciliata  and  in  the  ciliated  epithelial  cells  of  higher 
animals  is  much  more  difficult  to  determine.  It  was  held  by  Henneguy 
(1897),  Lenhossek  (1898),  Hertwig  (1902),  and  others  that  these  granules, 
like  the  basal  granules  of  flagella,  are  modified  centrosomes;  whereas 
certain  other  investigators  (Maier  1903,  Studnicka  1899,  Schuberg  1905) 
have  found  evidence  in  favor  of  a  contrary  interpretation.     An  extensive 


l- ....    /,         A    \ 

k\nM\ 


W* 


Fig.  35. — Spermatogenesis  in  Helix  pomatia, 
showing  growth  of  flagellum  from  outer  centro- 
some,  and  elongation  of  inner  centrosome  to 
form  axial  filament  of  middle  piece.  (After 
Korff,  1899.) 


Fig.  3(3. —  Diagram  of  a  ciliated  epi- 
thelial cell.  (Constructed  from  figures  of 
Saguchi,  1917.) 


discussion  of  this  question  is  given  by  Erhard  (1911),  who  concludes  that 
although  the  basal  corpuscles  arise  from  the  nucleus  in  a  manner  similar 
to  that  of  the  centrosomes  of  such  cells,  the  evidence  is  on  the  whole 
unfavorable  to  the  theory  of  Henneguy  and  Lenhossek. 

Still  more  recent  are  the  researches  of  Saguchi  (1917),  who  describes 
in  great  detail  the  insertion  of  the  cilia  in  epithelial  cells.  At  the  base 
of  each  cilium,  which  itself  shows  no  internal  structural  differentiation, 
there  is  always  a  basal  corpuscle  (Fig.  36).  These  corpuscles,  and  hence 
the  cilia,  are  in  parallel  rows;  and  beneath  each  row  there  is  a  transparent 
zone  in  which  the  rootlets  of  the  cilia  are  anchored,  and  through  which 
they  pass  and  become  continuous  with  strands  of  the  cytoplasmic  recti- 
culum.  Cilium,  corpuscle,  rootlet,  and  cytoplasmic  strand  form  one 
continuous  structure.  Saguchi  believes  that  neither  the  cilium  nor  the 
rootlet  causes  the  ciliary  movement,  but  that  the  kinetic  center  of  this 
movement  is  in  the  basal  corpuscles,  as  Henneguy  and  Lenhossek  thought. 
Contrary  to  the  opinion  of  those  authors,  however,  he  regards  the  ciliary 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST  97 

apparatus  as  entirely  independent  of  centrosomes,  holding  rather  thai  it 
is  produced  by  the  differentiation  of  chondriosonies,  and  thai  the  resem- 
blance of  ciliated  cells  to  spermatids,  in  which  centrosomes  do  produce 
the  motor  apparatus,  is  an  accidental  one. 

Conclusion. — In  conclusion  it  may  be  said  that  it  is  highly  probable 
that  cilia-bearing  structures  are  not  homologous  in  all  plant  and  animal 
groups.  It  is  beyond  question  that  in  animals  the  centrosomes  of  the 
spermatid  produce  the  motor  apparatus  of  the  spermatozoon.  That  a 
similar  interpretation  is  to  be  placed  upon  the  blepharoplasts  in  the  sper- 
matids (androcytes)  of  bryophytes,  pteridophytes,  and  gymnosperma 
appears  to  be  equally  well  demonstrated.  The  blepharoplasts  of  the 
flagellates  are  also  probably  centrosomic  in  nature,  at  least  in  certain 
cases.  In  the  " plasmodermal  blepharoplasts"  of  motile  alga  cells  we 
have  organs  which,  in  the  light  of  our  present  knowledge,  do  not  appear 
to  belong  to  the  centrosomic  category,  but  final  disposition  of  them  musl 
await  further  information  concerning  those  alga*  which  possess  both  cen- 
trosomes and  blepharoplasts.  It  can  scarcely  be  doubted  that  the  basal 
corpuscles  of  ciliated  cells  represent  organs  belonging  to  various  categori 
It  must  be  left  for  further  research  to  determine  just  how  far  thi 
structures,  which  are  functionally  analogous,  are  homologous  witli  each 
other  and  with  other  cell  organs. 

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Fraser,  H.  C.  1.     1908.     Contributions  t<>  the  cytology  of  Humaria  rutilans.     Ann. 

Bot.  22:  35-55.     pis.  4,  5. 
Fraser,  H.  C.  I.  and  Welsford,  E.  J.     1908.     Further  contributions  t<>  the  cytol< 

of  the  ascomycetes.     Ibid.  22:  465-477.     pis.  26,  27. 
Fraser,  H.  C.  I.  and  Brooks,  W.  E.  St.  J.     1909.     Further  studies  on  the  cytoli 

of  the  ascus.     Ibid.  23  :  538-549. 
Fujii,   K.     1898.     (Has  the  spermatozoid  of  Ginkgo  m    tail  or   not?)      Bot.    M 

Tokyo  12 :  287-290.     (Japanese.) 

1899.  (On  the  morphology  of  the  spermatozoid  of  Ginkgo  biloba.)     Ibid.  13:  '-'tin 
266.     pi.  7.     (Japanese.) 

1900.  (Account  of  a  sperm  with  two  spiral  bands.)      Ibid.  14:  16   17.      (Japan 
Graham,  M.     1918.     Centrosomes  in  fertilization  stages  of  Preissia  tiiiu<lr<i<i>    Scop. 

Nees.     Ann.  Bot.  32 :  415-420.     pi.  10. 
Gregoire,  V.  et  Berghs,  J.     1904.     La  figure  achromatique  dans  Le  Pellia  epiphylla. 

La  Cellule  21 :  193-238.     pis.  1,  2. 
Griggs,  R.  F.     1912.     The  development  and  cytology  of  Rhodochyirium.     Bot .  <  laz. 

53:  127-173.     pis.  11-16. 
Guignard,    L.     1889.     Developpement    et    constitution    des    antherozoides.     Rev. 

Gen.  Bot.  1:  11-27,  63-78,  136-145,  175-194.     pis.  2-6. 

1898.  Centrosomes  in  Plants.     Bot.  Gaz.  25:  158-164. 

Guilliermond,    A.     1904.     Recherches    sur  la   karyokinese  chez  les  ascomycfct< 

Rev.  Gen.  Bot.  16:  129-143.     pis.  14,  15. 
1905.     Rem arques  sur  la  karj'okinese  des  ascomycetes.     Ann.  Mycol.  3:  344   361 

pis.  10-12. 
1911.     Apercu  sur  revolution  nuclcaire  des  ascomycetes  et  nouvi-llcs  observations 

sur  les  mitoses  des  asques.     Rev.  Gen.  Bot.  23:  89-121.  figs.  8.  pis.   I.   5 
Harper,  R.  A.     1895.     Beitrag  zur  Kenntniss  der  Kernteilung  and  Sporenbildang 

im  Ascus.     Ber.  Deu.  Bot.  Ges.  13:  (67)-(68).     pi  27. 
1897.     Kerntheilung  und  freie  Zellbildung  im  Ascus.     Jahrb.  Wiss.  Bot.  30:  249 

284.     pis.  11,  12. 

1899.  Cell  division  in  sporangia  and  asci.     Ann.  Hot.  13:    167  525.     pis.  _' I   26. 
1905.     Sexual  reproduction  and  the  organization  of  the  uucleus  in  certain  mildews. 

Carnegie.     Inst.  Publ.  37.     Washington. 
1919.     The  structure  of  protoplasm.     Am.  Jour.  Bot.  6:  273  300. 
Hartmann,  M.  und  Noller,  W.     1918.     Untersuchungen  uber  die  Cytologie  von 

Trypanosoma  theileri.     Arch.  Protistenk.  38:  :i.")."»  :;7I      pis.   11.   15.  figs 
Henneguy,  L.  F.     1897.     Sur  les  rapports  des  cils  vibratiles  avec  les  centrosomi 

Arch.  d'Anat  Micr.  1:  481-496.  figs.  5. 
HiRAsfi,  S.     1894.     Notes  on  the  attraction  spheres  in  the  pollen  cells  of  Ginkgo  biloba. 

Bot.  Mag.  Tokyo  8:  359. 
Humphrey,  J.  E.     1894.     Nucleolen  und  Centrosomen.     Ber.   Deu.   Bot.  Ges.  12: 

108-117.     pi.  6. 
Humphrey,    H.    B.     1906.     The    development    of    Fossombronia    longiseta   Austr 

Ann.  Bot.  20:  83-108.     pis.  :..  6.     figs    8. 


100  INTRODUCTION  TO  CYTOLOGY 

Ikeno,  S.  1898.  Untersuchungen  fiber  die  Entwicklung  der  Geschlechtsorgane  und 
den  Vorgang  der  Befruchtung  bei  Cycas  revoluta.  Jahrb.  Wiss.  Bot.  32  :  557-602. 
pis.  8-10. 

1903.  Die  Spermatogenese  von  Marchantia  polymorpha.     Beih.  Bot.   Centralbl. 
15 :  65-88.     pi.  3. 

1904.  Blepharoplasten  im  Pflanzenreich.     Ibid.  24.  211-221.     figs.  1-3. 

1906.     Zur  Frage  nach  der  Homologie  der  Blepharoplasten.     Flora  96:  538-542. 
Jahn,    E.     Myxomycetenstudien.     3.     Kernteilung    und    Geisselbildung    bei    den 

Schwarmern  von  Stemonitis  flaccida  Lister.     Ber.  Deu.  Bot.  Gesell.  22 :  84-92. 

pi.  6 
Karsten,   G.     1900.     Die  Auxosporenbildung  der  Gattungen  Cocconeis,  Surirella, 

und  Cymatopleura.     Flora  87 :  253-283.     pis.  8-10. 
Koernicke,    M.     1903.     Der  heutige  Stand  der  pflanzlichen   Zellforschung.     Ber. 

Deu.  Bot,  Gesell.  21:  (66)-(134). 
1906.     Zentrosomen  bei  Angiospermen  ?     Flora  96:  501-522.     pi.  5. 
von   Korff,  K.  1899.     Zur   Histogenese   der   Spermien  von   Helix  pomatia.     Arch. 

Mikr.  Anat.  54:  291-296.     pi.    16. 
Kuczynski,  M.  H.     1917.     Ueber  die  Teilung  der  Trypanosomenzelle  nebst  Bemer- 

kungen  zur  Organization  einiger  nahestehender  Flagellaten.     Arch.  f.  Protistenk. 

38:94-112.     pis.  3,  4. 
Lauterborn,  R.     1896.     Untersuchungen  uber  Bau,   Kernteilung,   und  Bewegung 

der  Diatomen.     Leipzg. 
von  Lenhossek,  M.  1898.     Ueber  Flimmerzellen.     Verh.  Anat,  Ges.  Kiel  12:  106. 
Levine,  M.     1913.     The  cytology  of  Hymenomycetes,  especially  the  Boleti.     Bull. 

Torr.  Bot.  Club  40:  137-181.     pis.  4-8. 
Lewis,  C.  E.     1906.     Embryology  and  development  of  Riccia  lutescens  and  Riccia 

crystaUina.     Bot.  Gaz.  41 :  109-138.     pis.  5-9. 
Maier,  H.  N.     1903.     Ueber  die  feineren  Bau  der  Wimperapparate  der  Infusorien. 

Arch.  f.  Protistenk.  2. 
Maire,  R.     1905.     Recherches  cytologiques  sur  quelques  ascomycetes.     Ann.  Mycol. 

3:  123-154.     pis.  3-5. 
Mead,  A.  D.     1898.     The  origin  and  behavior  of  the  centrosomes  in  the  annelid  egg. 

Jour.  Morph.  14:  181-218. 
Meves,  F.     1897.     Ueber  Struktur  und  Histogenese  der  Samenfaden  von  Salamandra 

maculosa.     Arch.  Mikr.  Anat.  50:  110-141.     pis.  7,  8. 
1899.     Ueber  Struktur  und  Histogenese  der  Samenfaden  des  Meerschweinchens. 

Ibid.  54:  329-402.     pis.  19-21.     figs.  16. 
Meyer,  K.     1911.     Untersuchungen  liber  den  Sporophyt  der  Lebermoose.     I.     Ent- 

wicklungsgeschichte  des  Sporogons  der  Corsinia  marchantioides.     Bull.  Soc.  Imp. 

Moscou  236-286. 
Minchin,  E.  A.     1912.     An  Introduction  to  the  Study  of  the  Protozoa.     London. 
Miyake,  K.     1905.     On  the  centrosome  of   Hepaticse.     Bot,  Mag.  Tokyo  19:  98- 

101. 
1906.     The    spermatozoid    of    Ginkgo.     Jour.    Appl.  Micr.  and  Lab.  Methods  5: 

1773-1780.     figs.  10. 
Moore,  J.  E.  S.     1895.     Structural  changes  in  the  reproductive  cells  during  spermato- 
genesis of  elasmobranchs.     Quar.  Jour.  Micr.  Sci.  38:  275-313.     pis.  13-16. 
Morgan,  T.  H.     1896.     The  production  of  artificial  astrospheres.     Arch.  Entw.  3: 

339-361.     pi.  19. 
1899.     The  action  of  salt  solutions  on  the  unfertilized  and  fertilized  eggs  of  Arbacia 

and  other  animals.     Ibid.  8:  448-539.     pis.  7-10.     figs.  21. 
Mottier,  D.   M.     1898.     Das  Centrosom  bei  Dictyota.     Ber.  Deu.  Bot,   Ges.   16: 

123-128.     figs.  5. 


THE  CENTROSOME  AND  THE  BLEPHAROPLAST  101 

1900.      Nuclear  and  cell  division  in  Diclyota  dichotoma.      Ann.   Bot.  14;   166    L92 

pi.  11. 
1904.     The  development  of  the  spermatozoid  of  Chora.     Ibid.  18:245  _'.">}.     pi.  17. 
Paulmier,  F.  C.     1899.     The  spermatogenesis  of  Anasa  trUtis.     Jour.  Morph.  15: 

Suppl.  223-272.     pis.  13,  11. 
Rawitz,  B.     1896.     Untersuchungen  iiber  Zelltheilung.     I.     Arch.  Mikr.  Anat.  47: 

159-180.     pi.  11. 
Saguchi,  S.     1917.     Studies  on  ciliated  cells.     Jour.  Morph.  29:  217  L'T'.i.     pis.  1    1 
Sands,    M.    C.      1907.     Nuclear   structure    and    spore    format  ion    in    Microsphcera. 

Trans.  Wis.  Acad.  Sci.  15:  733-752.     pi.  46. 
Sapehin,  A.  A.     1913.     Untersuchungen  iiber  die  Individuality  der  Plastide.     Ber. 

Deu.  Bot.  Ges.  31:  14-66.     fig.  1. 
Schaffner,  J.  H.     1908.     The  centrosomes  of  Marchantia  polymorpha.     Ohio  Na1 

9.     363-388. 
Schottlander,  P.     1893.     Beitrage  7ur  Kenntniss  des  Zellkerns  und  der  Sexual- 

zellen  bei  Kryptogamen.     Cohn's  Beitr.  Biol.  Pflanzen  6:  267-304.     pis.  4,  ">. 
Sharp,  L.  W.     1912.     Spermatogenesis  in  Equisetum.     Bot.  Gaz.  54:  89-119.     pis. 

7,8. 
1914.     Spermatogenesis  in  Marsilia.     Ibid.  58:  419-431.     pis.  33,  34. 
1920.     Spermatogenesis  in  Blasia.     Ibid.  69:  258-268.     pi.   15. 
Shaw,  W.  R.     1898.     Ueber  die  Blepharoplasten  bei  Onoclea  und  Marsilia.     Ber. 

Deu.  Bot.  Ges.  16:  177-184.     pi.  11. 
Smith,  H.  L.     1886-1887.     A  contribution  to  the  life  history  of  the  Diatomaces. 

Proc.  Am.  Soc.  Micr.     Pts.  I  and  II. 
Strasburger,   E.     1892.     Schwarmsporen,   Gameten,   pflanzliche   Spermatozoiden, 

und  das  Wesen  der  Befruchtung.     Hist.  Beitr.  4 :  49-158.     pi.  3. 
1897.     Kerntheilung  und  Befruchtung  bei  Fucus.     Jahrb.  Wiss.  Bot.  30:351-374. 

pis.  27,.  28. 

1900.  Ueber  Reduktionstheilung,  Spindelbildung,  Centrosomen,  und  Cilienbildnei 
im  Pflanzenreich.     Hist.  Beitr.  6:  1-224.     pis.  1-4. 

Studnicka,    F.    K.     1899.     Ueber   Flimmer-   und    Cuticularzellen    init    besonderei 

Beriicksichtigung  der  Centrosomenfrage.     Sitz-Ber.  K.  Bohmisch.     Ges.  Wiss. 

Math.-Naturwiss.  Classe,  35. 
Swingle,  W.  T.     1897.     Zur  Kenntniss  der  Kern-  und  Zelltheilung  bei  deD  Sphacela- 

riaceen.     Jahrb.  Wiss.  Bot,  30:  296-350.     pis.  15,  16. 
Thom,  C.     1899.     The  process  of  fertilization  in  Aspidium  and  Adiantum.     Trans. 

Acad.  Sci.     St.  Louis  9:  285-314.     pis.  36-38. 
Timberlake,   H.  G.     1902.     Development  and  structure  <>t"  the  swarm  sporea  of 

Hijdrodictyon.     Trans.  Wis.  Acad.  Sci.  13:  486-522.     pis.  29.  30. 
Van  Hook,  J.  M.     1900.     Notes  on  the  division  of  the  cell  and  nucleus  in  Liverworts, 

Bot.  Gaz.  30:  394-399.     pis.  2,  3. 
Webber,  H.  J.      1897a.     Peculiar  structures  occurring  in  the  pollen  tube  of  Zatnia. 

Bot.  Gaz.  23:  453-459.     pi.  40. 
18976.     The  development  of  the  antherozoid  of  Zamia.     Ibid.  24:  16  22.     figs.  5. 
1897c.     Notes  on  the  fecundation  of  Zamia  and   the   pollen   lube  apparatus  of 

Ginkgo.     Ibid.  24:  225-235.     pi.  10. 

1901.  Spermatogenesis  and   fecundation    in  Zamia.      U.   S.  Dept.  Agr.  Pit.  [nd. 
Bull.  2.     pp.  100.     pis.  7. 

Williams,  J.  L.     1904.     Studies  in  the  Dictyotaceae.     II.  The  cytology  of  the  game- 
tophyte  generation.     Ann.  Bot.  18:  L83  204.      pis.  12    L4. 

Wilson,  E.  B.      1900.      The  Cell  in  Development  ami  Inheritance.      (p.   175.) 

1901.     Experimental  Studies  in  Cytology.     I.  A  cytological  study  of  partheno- 
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102  INTRODUCTION  TO  CYTOLOGY 

Wilson,  M.     1911.     Spermatogenesis   in  the  Bryophyta.     Ann.  Bot.  25:  415-457. 

pis.  37,  38.     figs.  3. 
von  Winiwarter,  H.     1912.     Observations  cytologiques  sur  les  cellules  interstiti- 

elles  du  testicule  humaine.     Anat.  Anz.  41:  309-320.     pis.  1-2. 
Wolfe,  J.  J.     1904.     Cytological  studies  on  Nemalion.     Ann.  Bot.   18:  607-630. 

pis.  40,  41.     fig.  1. 
Woodburn,   W.  L.     1911.     Spermatogenesis  in  certain  Hepaticae.     Ann.   Bot.   25: 

299-313.     pi.  2:». 
1913.     Spermatogenesis  in  Blasia  pusilla.     Ibid.  27:  93-101.     pi.  11. 
1915.     Spermatogenesis  in  Mnium  affine,  var.   ciliaris   (Grev.),   CM.   Ibid.   29: 

441-456.     pi.  21. 
Yamanouchi,  S.     1906.     The  life  history  of  Polysiphonia  violacea.     Bot.  Gaz.  42: 

401-44S.     pis.  19-28. 

1908.  Spermatogenesis,   oogenesis,  and   fertilization   in   Nephrodiwm.     Ibid.  45 : 
145-175.     pis.  6-8. 

1909.  Mitosis  in  Fucus.     Ibid.  47  :  173- 197.     pis.  8-11. 

Zimmermann,  A.  1893-1894.  Sammel-Referate.  6.  Die  Centralkorper  und  die 
Kerntheilung.  12.  Die  Cilien  und  Pseudocilien.  Beih.  Bot.  Centralbl.  3: 
342-354;  4:  169-171. 


CHAPTER  VI 

PLASTIDS  AND  CHONDRIOSOMES 

PLASTIDS 

Next  to  the  nucleus,  the  most  conspicuous  organ  held  within  the 
cytoplasm  of  the  plant  cell  is  the  plastic!.  Cytologists  have  long  been 
aware  of  the  important  plrysiological  roles  played  by  plastids  of  various 
types  in  the  life  of  the  cell,  but  it  is  only  recently  that  an  added  Inter- 
est has  been  given  these  organs  by  the  discovery  that  certain  peculiar 
characters  showing  definite  modes  of  inheritance  are  closely  bound  up 
with  their  behavior.  Such  problems  are  complicated  by  the  relation 
apparently  borne  by  plastids  to  chondriosomes.  In  the  present  chap- 
ter will  be  set  forth  some  of  the  more  important  facts  regarding  thi 
two  classes  of  cell  elements. 

General  Nature  and  Occurrence.— Plastids  are  differentiated  port  ions 
of  the  protoplasm,  as  von  Mohl  long  ago  pointed  out,  and  represent 
regions  in  which  certain  processes  have  become  localized  (Harper  1919). 
In  view  of  their  power  of  growth  and  division  and  their  definite  relation  to 
certain  important  physiological  functions  they  are  to  be  regarded 
distinct  cell  organs. 

Although  plastids  can  be  found  in  the  cells  of  both  animals  and  plants 
they  are  chiefly  characteristic  of  the  latter,  where  they  are  presenl  in 
one  form  or  another  in  all  groups  with  the  possible  exception  of  bacteria. 
myxomycetes,  and  certain  fungi.  They  are  abundant  only  in  th<  se  plant 
parts  which  have  to  do  with  specialized  physiological  functions.  Within 
a  single  cell  there  may  be  regularly  but  one  plastid,  as  in  many  a'gse, 
Anthoceros,  and  the  meristematic  cells  of  Selaginella  (Haberlandl  L888, 
1905);  or  two,  as  in  Zygnema;  or  a  ligher  number,  as  in  the  green  tissues 
of  most  higher  plants.  They  lie  imbedded  in  the  cytoplasm  and  are 
often  closely  associated  with  the  nucleus;  they  are  never  found  normally 
in  the  vacuole.  1  he  positions  which  the}-  assume  within  the  cell  are  fre- 
quently related  in  a  definite  manner  to  certain  external  conditions:  in 
the  palisade  cells  of  green  leaves,  for  example,  the  chloroplasts  are  found 
near  the  upper  surface  if  the  incident  light  is  weak,  whereas  they  react  to 
strong  illumination  by  taking  up  less  exposed  positions  along  the  lateral 
walls. 

Plastids  may  be  conveniently  classified  on  the  basis  of  their  contained 
coloring  matters.  This  difference  in  color,  however,  is  secondary  in 
importance;  the  fundamental  distinction  is  that  based  upon  the  kind  of 

103 


104 


INTRODUCTION  TO  CYTOLOGY 


physiological  work  being  done,  the  various  pigments  being  associated  in 
an  intimate  manner  with  different  reactions  occurring  within  the  plastids. 

Leucoplasts. — Leucoplasts  are  relatively  small  and  colorless.  They 
are  found  commonly  in  the  cells  of  meristematic  tissue,  and  may  be 
retained  in  some  kinds  of  differentiated  cells,  such  as  the  glandular  hairs  of 
Pelargonium.  Kuster  (1911)  states  that  the  leucoplasts  of  Orchis  are 
very  fluid  in  consistency,  undergoing  amoeboid  changes  of  shape  and 
multiplying  by  irregular  fission.  The  smaller  leucoplasts  appear  to 
represent  juvenile  stages  in  the  development  of  plastids  of  more  highly 
differentiated  types,  for  under  certain  conditions  they  develop  into  the 
larger  and  more  highly  specialized  leucoplasts  known  as  amyloplasts,  and 
into  the  various  kinds  of  chromatophores  mentioned  below. 

Chromatophores. — Chromatophores,  or  chromoplasts,  are  plastids  bear- 
ing one  or  more  pigments,  and  having  thus  a  more  or  less  decided 
color.     In  green  plants  the  most  important  of  these  pigments  are  chlo- 


Fig.  37. — Various  forms  of  plastids. 
A,   Drapamaldia.     B,   Spirogyra.     C,   Anthoccros.     D,   chromoplasts  of  Ariscema.     E, 
cell  of  Selaginella,  showing  position  assumed  by  plastid  in  response  to  light  (direction  shown 
by  arrow).     A,  B,  and  C  show  pyrenoids.      (E  After  Haberlandt.) 


rophyll,  carotin,  and  xanthophyll.  Chlorophyll  is  apparently  a  combi- 
nation of  two  simpler  pigments,  chlorophyll  a  and  chlorophyll  b.  The 
cells  of  the  Phseophycese,  Cyanophycese,  and  Rhodophycese  are  character- 
ized respectively  by  the  presence  of  yellow  carotin,  blue  phycocyanin, 
and  red  phycoerythrin,  in  addition  to  chlorophyll.  The  Cyanophycese 
exhibit  an  especially  rich  variety  of  pigments,  which  m  many  cases  do  not 
appear  to  be  held  within  definite  chromatophores.1 

Chromatophores  are  usually  spherical,  ovoid,  or  discoid  in  shape,  but 
many  peculiar  forms  are  known,  particularly  among  the  green  algae.  In 
Ulothrix  the  chloroplast  has  the  form  of  a  complete  or  incomplete  hollow 
cylinder;  in  Drapamaldia   (Fig.   37,  A),   sl  hollow   cylinder  with  veiy 

1  For  the  literature  pertaining  to  plant  pigments  see  Palladin  1918.  See  also 
Haas  and  Hill  (1913),  Willstatter  and  Stoll  (1913),  Jorgensen  and  Stiles  (1917), 
Wheldalc  (1916),  and  Beauverie  (1919).  The  distribution  of  carotin  is  discussed  in 
an  earlier  paper  by  Tammes  (1900). 


PLASTIDS  AND  CH0NDRI0S0M1  105 

irregular  v\n\s;  in  Mdogonium,  an  irregular  parietal  net;  in  Spirogyra 
(Fig.  37,  #),  a  spirally  coiled  ribbon;  and  in  the  desmids,  a  series  of 
radiating  plates  (Carter  1919,  1920).  The  chromatophore  of  Antho- 
ceros  (Fig.  37,  C)  is  spindle-shaped,  becoming  chain-like  in  the  elongated 
columella  colls  (Scherrer  1914).  The  chromatophore  of  Selaginella 
may  also  assume  this  form  (Haberlandt  1888).  The  chromoplasts  of 
Ariscema  (Fig.  37,  D)  are  frequently  sharply  angular.  In  the  Clado- 
phoraceae  (Carter  1919)  the  cell  is  completely  lined  by  a  thin  chromato- 
phore which  maybe  entire  or  fenestrated.  In  many  cells  irregular  strands 
pass  inward  through  the  cell  cavity.  Indeed  it  seems  not  improbable 
that  in  some  such  cases  the  plastid  may  be  not  at  all  sharply  distinct 
from  the  rest  of  the  cytoplasm,  the  two  grading  one  into  the  other,  and 
the  chlorophyll  at  certain  stages  permeating  all  parts  of  the  cytoplasm. 
The  observations  of  Timberlake  and  Harper  appear  to  show  that  such  is 
the  condition  in  the  young  cells  of  Hydrodictyon.  Thus  the  physiological 
processes  show  various  degrees  of  localization  in  the  cell,  causing  manifold 
degrees  of  structural  transformation  and  delimitation  of  the  cytoplasmic 
regions  involved  (Harper). 

Of  all  chromatophores  the  chloroplasts  stand  first  in  importance,  for 
they  bear  the  green  pigment,  chlorophyll,  which,  in  the  presence  of 
light,  enables  them  to  combine  water  from  the  soil  or  other  surrounding 
medium  with  carbon  dioxide  from  the  atmosphere  to  form  carbohydrates, 
the  first  visible  product  being  starch.  The  chloroplasts  are  therefore 
the  world's  ultimate  food  producers.  In  addition  to  chlorophyll  other 
pigments,  notably  xanthophyll,  are  usually  present.  Although  the  body 
of  the  chloroplast  can  be  developed  in  darkness,  the  chlorophyll  will 
usually  not  be  elaborated  unless  light  is  present.  Most  young  seedlings 
grown  in  the  absence  of  light  show  a  pale  yellowish  color,  which  is  due  to  a 
substance  known  as  chlorophyllogen,  contained  in  the  plastids.  When 
such  "etiolated"  plants  are  placed  in  the  light  the  plastids  become  green. 
apparently  through  an  alteration  of  the  chlorophyllogen  to  chlorophyll 
(Monteverde  and  Lubimenko  1911).  Other  conditions  necessary  for  the 
development  of  chlorophyll  are  a  favorable  temperature  and  the  presence 
of  iron,  oxygen,  and  certain  carbohydrates. 

The  structure  of  the  chloroplast  is  an  extremely  difficult  matter  to 
determine,  and  has  been  the  subject  of  some  controversy.  It  is  generally 
thought  that  the  body  of  the  plastid  is  composed  of  a  finely  fibrillar 
meshwork,  the  stroma,  which  may  be  somewhat  denser  at  the  periphery, 
and  that  the  coloring  matters  are  held  in  the  meshes  of  the  Btroma  in  the 
form  of  minute  droplets.  No  limiting  membrane  is  definitely  known. 
The  included  droplets  are  apparently  not  composed  of  the  pigments  alone: 
it  is  probable  that  they  are  rather  globules  of  some  oily  or  fatty  material 
containing  the  pigments  in  solution.  The  pigments  may  easily  be  dis- 
solved out  with  alcohol  and  other  reagents.     On  the  other  hand,  it  has 


106 


INTRODUCTION  TO  CYTOLOGY 


been  held  by  some  observers  that  the  stroma  is  a  homogeneous  body  in 
which  the  droplets  of  chlorophyll  solution  are  imbedded,  and  that  the 
reticular  structure  so  often  reported  is  an  artifact  due  to  the  reagent 
employed  in  removing  the  chlorophyll.  By  others  the  pigment  has  been 
thought  to  form  a  layer  about  the  plastid.  In  any  case  it  seems  evident 
that  the  chlorophyll  is  not  uniformly  distributed  throughout  the  stroma. 
In  chromatophores  other  than  chloroplasts  the  pigments  may  at  times 

take  the  form  of  solid  granules  or  crystals. 

Starch. — After  a  period  of  photosynthetic 
activity  the  chloroplast  contains  starch,  the  first 
visible  product  of  that  activity,  in  the  form  of 
minute  granules.  This  "assimilation  starch"  is 
formed  within  the  body  of  the  chloroplast,  as 
Meyer  originally  showed  (Fig.  38,  A,  B).  It  is 
later  transformed  through  the  agency  of  enzymes 
into  some  soluble  compound,  usually  a  sugar;  in 
this  form  it  may  be  carried  to  growing  regions, 
where,  after  further  changes,  it  is  built  into  the 
structure  of  the  plant.  Or,  it  may  pass  to  storage 
organs  where  it  is  transformed  into  the  ordinary 
"reserve  starch,"  or  "storage  starch."  This  de- 
position of  reserve  starch  is  brouj;  ht  about  through 
the  agency  of  a?nyloplasts,  which  are  leucoplasts 
capable  of  changing  already  elaborated  organic 
materials,  such  as  glucose,  into  starch  (Fig.  38,  C). 
Reserve  starch,  upon  which  we  depend  so 
largely  for  food,  is  a  carbohydrate  with  a  composi- 
tion expressed  by  the  general  formula  (C6Hio05)n, 
and  exists  in  the  form  of  granules  ranging  in  size 
approximately  from  2//.  to  200//,  in  different  plants. 
Potato-starch  grains  are  usually  about  90/.;  in 
diameter.  The  reserve  starch  grain  is  formed 
within  the  body  of  the  amyloplast,  and  is  made 
up  of  a  series  of  concentric  layers  successively 
laid  down  about  a  center,  or  "hilum"  (Fig.  39,  A).1  In  case  the  grain 
starts  to  form  near  the  middle  of  the  amyloplast  it  may  develop  sym- 
metrically, but  commonly  the  developing  grain  lies  near  the  periphery 
of  the  amyloplast,  which  becomes  greatly  distended  as  the  grain  grows. 
Material  is  thus  deposited  unevenly  upon  the  grain  so  that  the  latter 
becomes  very  eccentric;  in  extreme  cases  the  grain  ruptures  the  amylo- 
plast and  remains  in  contact  with  it  only  at  one  side,  where  all  new 
material  is  then  deposited.     Several  grains  may  start  to  develop  simul- 

1  For  the  structure  of  the  starch  grain  see  the  papers  of  Nageli,  Schimper,  Meyer, 
Binz,  Dodel,  Salter,  and  Kramer. 


Fig.  38. — Formation  of 
starch  by  plastids. 

A,  dividing  chloro- 
plasts of  Funaria,  with 
grains  of  assimilation 
starch.  X  940.  (After 
Strasburger.)  B,  chloro- 
plast of  Zygnema,  with 
several  large  starch 
grains  about  a  central 
pyrenoid.  (After  Bour- 
quin,  1917.)  C,  leuco- 
plast  (amyloplast)  in 
aerial  tuber  of  Phajus 
grandifolius  with  grain 
of  reserve  starch.  (After 
Strasburger.) 


PLASTIDS  AND  CHONDRIOSOMES 


107 


taneously  in  a  single   amyloplast,   later  growing  together   to   form   a 

"compound  grain"  with  more  than  one  hilum.  In  case  the  parts  making 
up  the  compound  grain  are  enveloped  in  one  or  more  common  outer 
layers  the  grain  is  said  to  be  "half-compound."  Potato  March  is  mad.' 
up  of  simple,  compound,  and  half-compound  grains,  whereas  in  ..al- 
and rice  all  or  nearly  all  of  the  grains  are  said  to  be  of  the  compound  type. 
The  successively  deposited  Jayers  making  up  the  grain  differ  mainly  in 
water  content,  the  innermost  layers  being  richesl  and  the  outermosl 
poorest  in  water.  As  a  result  of  this  non-uniform  composition  the  main 
often  splits  radially  when  dried. 


-*a4 


Fig.  39. — Reserve  starch  grains  from  various  plant-. 

A,  potato;  simple  and  half-compound  grains.  B,  Colombo  starch. 
D,  pea.  E,  maize;  intact  and  partially  digested  grains.  F.  rye.  G,  maize. 
/,  bean.     J,  rice.     K,  wheat.      (After  Tschirch.) 


c.    arrowroot. 
H,  Euphorbia 


As  a  result  of  his  classic  researches  Nageli  (1858)  advanced  t  he  theory 
that  the  starch  grain  is  made  up  of  ultramicroscopic  crystalline  particles 
which  he  called  "micellae/'  these  being  surrounded  by  water  films  of 
varying  thickness.  It  was  similarly  held  by  A.  Meyer  L883,  L895 
that  the  grain  is  composed  of  radially  arranged  needle-shaped  crystals 
known  as  "trichites;"  these  are  composed  of  a-  and  0-amylose  which 
turn  blue  with  iodine.  In  some  starch  amylodextrin  and  dextrin  are 
also  present  such  grains  turning  red  with  iodine.  Both  Nageli  and  Meyer 
held  the  stratification  of  the  grain  to  be  due  to  the  varying  numbers  of 
the  crystalline  units  in  the  successive  layers,  and  Meyer  showed  that  in 
certain  cases  it  is  correlated  with  the  alternation  of  day  and  night,  and 
therefore  with  a  periodic  activity  on  the  pari  of  the  plastid.  This  eon- 
elusion  was  confirmed  by  Salter  (1898). 

The  statement  made  by  Schimper  (1880)  and  Meyer  (1883,  L895)  that 
starch  is  always  formed  by  plasl  ids  st  ill  holds  good  in  it  -  essent  ial  feature : 
so  far  as  is  certainly  known  no  primary  product  of  photosynthesis  is 
formed  in  the  cytoplasm  apart  from  plastids,  although  in  some  cas< 
such  as  the  young  cells  of  Hydrodictyon,  according  to  Harper,  it  is  very 
difficult  or  even  impossible  to  distinguish  the  limits  of  these  organs      The 


108  INTRODUCTION  TO  CYTOLOGY 

granules  of  paramylum  in  Euglena  and  those  of  "Floridean  starch'    in 

the  red  alga?  first  appear  in  the  cytoplasm;  but,  although  they  are  the 
first  substances  which  are  visible,  it  is  highly  probable  that  they  arise 
through  the  transformation  of  a  non-visible  product  (sugar?)  of  the 
photosynthetic  activity  of  the  plastids,  and  are  not  immediately  built 
up  from  water  and  carbon  dioxide.  A  similar  interpretation  may  be 
placed  upon  corresponding  appearances  reported  in  the  case  of  higher 
plants.  Owing  to  the  great  difficulty  of  determining  the  true  cell  struc- 
ture of  the  Cyanophyeeae  (see  p.  202)  it  is  possible  to  speak  of  plastid 
activity  in  such  forms  only  with  great  reserve.  If,  as  Olive  (1904)  and 
Gardner  (1906)  hold,  these  cells  are  without  plastids,  the  product  of 
photosynthetic  activity,  commonly  glycogen,  must  be  elaborated  in  the 
cytoplasm  without  their  aid.  If,  on  the  other  hand,  the  peripheral 
portion  of  the  protoplast  represents  a  large  chromatophore  (Fischer  1898), 
or  cytoplasm  containing  a  large  number  of  minute  chromatophores 
(Hegler  1901,  Kohl  1903,  Wager  1903),  the  photosynthetic  process, 
although  it  may  result  in  the  production  of  a  different  substance,  is 
dependent  upon  the  powers  of  definite  protoplasmic  organs  much  the 
same  as  in  higher  plants.  Among  bacteria  and  other  low  forms  in  which 
it  seems  more  certain  that  plastids  and  the  ordinary  pigments  are  absent, 
widely  different  types  of  metabolism  are  met  with.  For  further  discus- 
sion of  this  subject,  which  lies  outside  the  scope  of  the  present  book, 
more  special  physiological  works  should  be  consulted. 

The  Pyrenoid. — The  term  pyrenoid  was  applied  by  Schmitz  (1882) 
to  the  refractive  kernel-like  bodies  imbedded  in  the  chromatophores  of  the 
algae.  Pyrenoids  are  characteristic  of  the  Chlorophyceae  especially, 
being  present  almost  universally  in  the  members  of  this  group.  They 
are  known  in  a  few  representatives  of  the  IJiodophycese  (Nemalion 
and  the  Bangiaceae),  but  apparently  do  not  occur  in  the  cells  of  the 
Cyanophyceae,  Phseophycese,  and  Characeae.  Very  rarely  they  are 
present  in  forms  above  the  algae :  a  conspicuous  example  is  the  liverwort 
Anthoceros.  The  chromatophore  may  contain  but  one  pyrenoid,  as  in 
Zygnema,  or  a  larger  number,  as  in  Spirogyra,  Draparnaldia,  and  many 
other  forms  (Fig.  37). 

As  held  by  de  Bary  (1858),  Schmitz  (1884),  and  Schimper  (1885), 
the  pyrenoid  appears  to  be  composed  of  a  protein  substance  with  a  thick 
gelatinous  consistency.  When  a  single  pyrenoid  is  present  in  the  chroma- 
tophore it  may  multiply  by  fission  along  with  the  latter  when  the  cell 
divides,  while  in  those  forms  possessing  several  pyrenoids  this  multiplica- 
tion may  be  much  more  extensive.  Also,  as  pointed  out  by  Schmitz 
and  Schimper,  and  more  recently  by  Smith  (1914),  the  pyrenoid  may 
disappear  and  arise  de  novo  from  the  cytoplasm  or  from  the  plastid 
protoplasm. 

With  regard  to  its  function,  the  early  workers  referred  to  above  ob- 


PLASTIDS  AND  CHONDRJOSOMES  109 

served  that  under  certain  conditions  the  pyrenoid  is  closely  surrounded 
by  amass  of  starch  grains,  and  concluded  thai  it  is  an  organ,  or  portion  of 
an  organ  (chromatophore),  intimately  concerned  in  the  process  of  starch 
formation,  its  action  being  somewhat  similar  to  thai  of  the  amyloplast. 

The  pyrenoid,  in  fact,  has  often  been  likened  to  a  leucoplast  imbedded  in 
the  chromatophore;  Wiesner,  for  instance,  believed  the  pyrenoid  t<> 
contain  several  leucoplast  bodies,  each  of  which  gave  rise  to  a  starch 
grain.  In  general,  more  recent  researches  have  emphasized  the  close 
association  of  the  pyrenoid  with  the  starch  forming  process,  although 
the  precise  nature  of  this  process  remains  very  much  in  doubt.  Accord- 
ing to  Timberlake  (1901)  the  pyrenoid  in  Hydrodictyon  is  different iated 
from  the  cytoplasm  and  is  very  active  in  starch  production,  segments 
splitting  off  from  its  periphery  and  forming  starch  within  them.  In 
this  way  the  entire  pyrenoid  may  become  a  mass  of  "pyrenoid  starch,'' 
as  distinguished  from  ordinary,  or  "stroma  starch."  McAllister  (1913) 
describes  a  similar  splitting  up  of  the  pyrenoid  to  form  several  starch 
grains  in  Tetraspora.  Yamanouchi  (1913),  however,  in  his  description 
of  a  new  species  of  Hydrodictyon,  states  that  some  of  the  chloroplasts 
give  rise  to  starch  while  others  give  rise  to  pyrenoids,  and  that  the 
latter  have  nothing  to  do  with  starch  formation. 

A  similar  diversity  of  opinion  exists  with  respect  to  the  role  of  the 
pyrenoid  in  Zygnema.  Chmielewskij  (1896),  who  looked  upon  the 
pyrenoid  as  a  permanent  cell  organ  multiplying  only  by  division,  held 
that  starch  grains  arise  wholly  from  the  substance  of  the  pyrenoid, 
plate-like  extensions  of  the  latter  being  present  between  and  in  intimate 
contact  with  the  developing  grains.  More  recently  Miss  Bonn  pun 
(1917)  asserts  that  the  pyrenoid  has  nothing  to  do  with  the  appearance 
of  starch,  the  body  of  the  chromatophore  alone  being  concerned.  She 
observes  the' starch  grains  appearing  first  near  the  periphery  of  the 
chromatophore  entirely  apart  from  the  pyrenoid,  the  later  formed  grains 
differentiating  in  positions  progressively  nearer  the  pyrenoid  (Fig.  38,  B). 

The  pyrenoid  of  Anthoceros  (Fig.  37,  C)  as  described  by  McAllister 
(1914)  is  in  reality  a  group  of  about  25-300  small  "pyrenoid  bodies1 
which  are  probably  composed  of  a  protein  substance.  The  outermosl 
bodies  become  starch,  new  ones  apparently  being  formed  by  the  fission 
of  those  lying  on  the  interior  of  the  group.  McAllister  states  thai  no 
pyrenoid  is  visible  in  the  young  sporogenous  tissue,  starch  being  formed 
without  its  aid.  Somewhat  later  several  small  bodies  appear  and 
aggregate  to  form  the  pyrenoid. 

Cleland  (1919)  recently  reports  a  close  association  of  the  pyrenoid 
of  Nemalion  with  the  formation  of  Floridean  starch. 

Elaioplasts  and  Oil  Bodies.— In  18SS  Wakker  discovered  in  the  cells 
of  Vanilla  planifolia  and  l\  aromatica  certain  plastid-like  bodies  to 
which  he  gave  the  name  elaioplasts,  since  they  seemed  to  be  concerned  in 


110 


INTRODUCTION  TO  CYTOLOGY 


mj^ 


the  elaboration  of  oil  (Fig.  40,  A).  They  were  soon  observed  in  a  number 
of  monocotyledons  by  Zimmermann  (1893),  Raciborski  (1893),  and  Kris- 
ter (1894);  and  some  time  later  in  the  flower  parts  of  a  dicotyledon, 
Gaillardia,  by  Beer  (1909).  Politis  (1914)  has  found  them  in  monocoty- 
ledonous  plants  belonging  to  19  different  genera,  and  in  five  genera  of 
dicotyledons  (Malvaceae). 

There  is  a  considerable  lack  of  agreement  in  the  opinions  expressed 
on  the  subject  of  the  origin  and  significance  of  elaioplasts.     Wakker 

thought  it  probable  that  they  represent  meta- 
morphosed chloroplasts,  which  they  often  closely 
resemble  in  structure  (Klister),  whereas  Raciborski 
asserted  that  they  arise  as  small  refractive  gran- 
ules in  the  cytoplasm  and  multiply  by  budding. 
In  the  zygospores  of  Sporodinia  grandis  and 
Phycomyces  nitens  Miss  Keene  (1914,  1919)  reports 
the  presence  of  a  number  of  globular  structures 
with  which  oil  is  associated  from  their  earliest 
stages.  These  unite  to  form  one  or  two  large 
reticulate  bodies  which  Miss  Keene  believes  are 
related  to  the  elaioplasts  of  higher  plants.  All  of 
these  investigators,  with  Politis,  agree  that  elaio- 
plasts are  normal  cell  organs  with  a  special  func- 
tion, namely,  the  formation  of  oily  substances 
having  a  role  in  nutrition.  Beer,  on  the  contrary, 
Fig.  40.  states  that  in  Gaillardia    they  are   formed  secon- 

A,  elaiopiast  forming    darily  by  the  aggregation   of   many  small  degen- 

oil  droplets  in  epidermal  1      x-  i  i     j.i  i 

cell  of  perianth  of  Poii-  ©rating  plastids  and  their  products  at  one  or 
anthes  tuberosa;  nucleus    more  points  in  the  cell,  all  stages   of  the   process 

with    small    plastids    at     ■,  i  i         \  i j.  1  1     .li        i       t  r  1 

right.  (After  Politis,  being  observed.  Although  the  bodies  so  formed 
1914.)    B,  oil  bodies  in    may,  if  green,  produce  starch,  or,   if  colorless,  an 

various  stages  of  develop-        -i  ^  •  ^  ,-.  .    ,       .,  ,     ,  ,      ,, 

ment  in  a  cell  of  Caly-  01iv  yellow  pigment,  Beer  thinks  it  probable  that 
pogeia.  (After Gar geanne,    they  have  no  important  special  function  in  the  life 

of  the  plant. 
Closely  associated  with  investigations  on  elaioplasts  have  been  those 
concerned  with  the  oil  bodies  found  in  the  cells  of  many  liverworts  (Fig. 
40,  B).  These  bodies,  discovered  by  Gottsche  in  1843,  were  first  carefully 
described  by  PfefTer  (1874).  Pfeffer  stated  that  they  arise  by  the  fusion 
of  many  minute  droplets  of  fatty  oil  appearing  in  the  cytoplasm  of  very 
young  cells,  and  later  come  to  lie  in  the  cell  sap;  he  further  believed  them 
to  possess  a  special  membrane.  Wakker  (1888)  held  them  to  be  analo- 
gous to  leucoplasts  and  chloroplasts,  multiplying  by  fission  at  each 
cell-division,  and  pointed  out  that  they  lie  in  the  cytoplasm  rather  than  in 
the  cell  sap.  He  was  inclined  to  view  them  as  products  of  elaioplasts, 
which  Kuster  (1894)  supposed  them  to  resemble  in  having  a  spongy 
stroma  containing  oil  in  the  form  of  minute  droplets. 


PLASTIDS  AND  CHONDRIOSOMES  1  1  1 

Quite  different  were  the  views  of  Gargeanne  (1903).  According  to 
him  they  arise  from  vacuoles,  their  limiting  membranes  thus  being  the 
original  tonoplasts.  While  in  the  juvenile  vacuole  stage  they  may  multi- 
ply by  division,  but  when  once  fully  formed  they  remain  unchanged  and 
divide  no  further.  Gargeanne  observed  small  oil  droplets  moving  aboul 
freely  within  the  oil  body,  and  hence  concluded  that  the  latter  has  a 
fluid  consistency  rather  than  a  spongy  stroma  as  Kiister  thought. 

The  most  noteworthy  recent  observations  on  oil  bodies  arc  those  of 
Rivett  (1918),  who  finds  them  to  be  very  conspicuous  in  the  cells  of 
Alicularia  scolaris.  Rivett  holds  that  they  are  in  reality  only  oil  vacuoles 
— that  they  originate  by  the  coalescence  of  numerous  minute  oil  droplets 
secreted  by  the  protoplasm  in  a  manner  entirely  similar  to  that  in  which 
the  ordinary  sap  vacuole  arises  (c/.  Pfeffer).  Although  they  become  very 
large  and  project  well  into  the  sap  vacuole,  they  continue  to  be  surrounded 
by  a  thin  film  of  cytoplasm.  The  oil  body,  in  the  opinion  of  Rivett,  is 
therefore  in  no  sense  a  plastid,  nor  is  it  formed  by  any  special  elaioplasl : 
it  is  simply  an  accumulation  of  ethereal  and  fatty  oils  together  with  souk 
protein  substance.  The  " membrane"  observed  by  Pfeffer  is  the  limiting 
layer  of  the  surrounding  cytoplasm,  which  may  be  slightly  changed  by 
contact  with  the  oil. 

Accumulations  of  oil  apparently  quite  similar  to  those  in  liverwort 
cells  have  been  described  in  the  cells  of  various  anriosperms  by  a  number 
of  writers.     To  these  the  term  elaiospheres  was  applied  by  Lidforss  f  1X9:-! 

The  published  figures  of  elaioplasts  and  oil  bodies  in  many  cases 
bear  striking  resemblance  to  those  of  fat-  and  oil-secreting  chondrio- 
somes  (see  below),  and  it  is  not  improbable  that  the  problem  of  their 
origin  and  significance  will  be  brought  nearer  solution  by  further  studies 
of  the  latter  class  of  bodies. 

The  Eyespot. — The  so-called  eyespot  present  in  the  flagellate  cell  and 
in  the  zoospores  and  gametes  of  many  alga?  has  certain  characteristics 
in  common  with  plastids,  and  may  therefore  receive  consideration  here. 
This  body,  which  nearly  all  workers  agree  is  a  light-sensitive  organ,  is  an 
elongated  or  circular  and  flattened  structure  lying  in  the  anterior  region 
of  the  cell  (flagellates)  or  near  its  lateral  margin,  usually  in  close  associa- 
tion with  the  chromatophore  and  the  plasma  membrane.  (Overton 
1889;  Klebs  1883,  1892;  Johnson  1893;  Strasburger  1900;  Wollenweber 
1907,  1908).  With  respect  to  its  mode  of  origin,  it  has  been  variously 
reported  to  arise  de  novo  in  each  newly  formed  zoospore  in  several  green 
algae  (Overton);  to  develop  from  a  colorless  plastid  in  the  young 
antheridial  cell  in  the  case  of  the  spermatozoid  of  Funis  \  ( Juignard  L889  ; 
to  arise  as  a  differentiated  portion  of  the  plastid  in  the  zoospores  and 
gametes  of  Zanardinia  (Yamanouchi);  and  finally  to  multiply  by  fission 
at  the  time  of  cell-division  in  flagellates  (  Klebs  L892 

It  is  generally  agreed  that  the  eyespot  in  many  instance-  consists  of 


112 


INTRODUCTION  TO  CYTOLOGY 


a  finely  reticulate  stroma  in  which  an  oily  red  pigment  with  many  of  the 
characteristics  of  hsematochrom  is  held  in  the  form  of  minute  droplets  or 
granules  (Schilling  1891;  Klebs  1883;  Franze  1893;  Wager  1900;  Wollen- 
weber  1907,  1908)  (Fig.  41,  D).  As  shown  by  the  careful  researches  of 
Franze,  the  stroma  may  also  bear  one  or  more  refractive  inclusions, 
which  in  the  Chlamydomonadacese  and  Volvocacese  consist  of  starch,  and 

in  the  Euglenoidese  of  paramylum  (Fig.  41, 
E).  These  inclusions  were  thought  by 
Franze  to  increase  the  sensitivity  of  the  eye- 
spot  by  concentrating  the  light  at  certain 
points. 

The  eyespot  of  the  zoospore  of  Cladophora 
(Strasburger  1900)  appears  to  arise  as  a 
swelling  of  the  plasma  membrane,  and  consists 
of  an  external  pigmented  layer  beneath  which 
is  a  lens-shaped  mass  of  hyaline  substance 
(Fig.  41,  B).  InGonium  and  Eudorina  (Mast 
1916)  the  lens-shaped  portion  lies  outside  with 
the  cup-shaped  opaque  portion  beneath  it 
(Fig.  41,  A),  an  arrangement  strongly  sug- 
gesting the  primitive  eyes  of  certain  higher 
organisms.  In  neither  portion  could  any  finer 
structure  be  detected.  Mast  has  shown  that 
the  orientation  of  the  colony  is  brought  about 
through  changes  in  the  intensity  of  the  light 
falling  upon  the  light-sensitive  substance.  As 
the  unoriented  swimming  colony  rotates  on 
its  axis,  those  zooids  turning  away  from  the 
light  have  the  hyaline  portion  of  their  eye- 
spots  shaded  by  the  opaque  cup;  this  sudden 
reduction  in  the  amount  of  light  energy 
received  brings  about  an  increase  in  the 
activity  of  the  flagellar  of  those  zooids,  with 
the  result  that  the  colony  as  a  whole  turns 
more  directly  toward  the  .source  of  light. 

In  Euglena  viridis  the  morphological  con- 
nection between  the  eyespot  and  the  motor  apparatus  is  particularly 
close.  Here  Wager  (1900)  has  shown  that  the  eyespot,  which  is  a 
discoid  protoplasmic  body  containing  a  layer  of  large  pigment  droplets, 
is  situated  at  the  surface  bounding  the  oesophagus  in  close  contact  with  a 
swelling  on  one  of  the  basal  branches  of  the  flagellum  (Fig.  41,  C). 

In  general  it  may  be  concluded  that  the  eyespot  in  some  cases  bears  in 
its  structure,  and  to  a  certain  extent  in  its  evident  function,  such  a  close 
resemblance  to  the    ordinary    plastid    that    a  relationship  of   some  sort 


-Eyespots  of   various 
types. 

A,  zooid  of  Eudorina;  e,  eye- 
spot.  (From  Mast,  After  Grave.) 
B,  zoospore  of  Cladophora, 
(After  Strasburger,  1900.)  C, 
anterior  end  of  Euglena  viridis, 
showing  eyespot  at  surface  of 
oesophagus,  and  in  front  of  it  a 
swelling  on  one  root  of  the 
flagellum;  face  view  of  eyespot 
at  right,  showing  pigment  gran- 
ules. (After  Wager,  1900.)  D, 
eyespot  of  Euglena  velata. 
(After  Franze,  1893.)  E,  eye- 
spot  of  Trachelomonas  volvo- 
cina,  with  pigment  granules 
and  crystalloid  body.  (After 
Franze.) 


PLASTIDS  AND  CHONDRIOSOMES  11:; 

between  the  two  seems  highly  probable;  whereas  in  oilier  cases  annum,. 
Cladophora)  it  appears  to  represent  a  differentiation  of  the  ectoplast. 
It  is  more  than  likely  that  light-sensitive  organs  have  arisen  more  than 
once  in  the  evolution  of  the  lower  organisms,  and  thai  they  cannol  all 
be  placed  in  the  same  category. 

The  Individuality  of  the  Plastid. — It  was  believed  by  the  early 
observers,  notably  Schimper  (1883)  and  Meyer  (1883),  that  plaslids 
never  originate  de  novo  but  always  arise  from  preexisting  plastids 
by  division.  Fully  differentiated  plastids,  such  as  chloroplasts,  can 
readily  be  seen  multiplying  in  this  manner  in  growing  tissues  with  a 
frequency  sufficient  to  account  for  the  large  number  of  plastids  present 
in  mature  plant  parts.  Since  it  is  known,  however,  that  chloropla-i- 
and  other  differentiated  chromoplasts  may  arise  from  leucoplasts  through 
the  development  of  pigments  and  other  characters  in  the  latter,  and  also 
that  the  individual  plant  arises  from  sex  cells  or  a  spore  in  which  the 
plastids  are  usually  in  a  colorless  and  relatively  undifferentiated  state  t  he 
problem  of  the  individuality  of  the  plastid  is  mainly  one  of  determining 
whether  these  undifferentiated  plastids,  leucoplasts,  or  " plastid  primor- 
dia"  later  developing  into  chloroplasts  and  other  types  are  continuous 
through  the  critical  stages  of  the  life  cycle,  multiplying  only  by  division. 
or  arise  de  novo  as  new  differentiations  of  the  cytoplasm.  At  this  point 
we  may  review  certain  cases  in  which  the  plastid  has  been  followed 
through  gametogenesis  and  fertilization. 

In  Zygnema  (Kurssanow  1911)  each  vegetative  cell  contains  one 
nucleus  and  two  plastids,  all  of  which  divide  at  each  vegetative  cell- 
division.  In  sexual  reproduction  the  entire  protoplast,  with  its  nucleus 
and  two  plastids,  passes  through  the  conjugating  tube  as  a  'male' 
gamete  and  unites  with  a  similar  complete  protoplast  ("female"  gamete  I 
of  another  filament.  The  two  nuclei  fuse,  giving  the  primary  nucleus  of 
the  new  individual  (zygospore  nucleus),  while  the  two  plastid-  contrib- 
uted by  the  "male,:  gamete  degenerate,  leaving  the  two  furnished  by 
the  " female"  gamete  as  the  plastids  of  the  new  individual. 

In  Coleochcete  (Allen  1905)  each  vegetative  cell  and  gamete  has  one 
nucleus  and  one  plastid:  after  the  sexual  union  of  the  gamete  nuclei  the 
fertilized  egg  therefore  contains  one  nucleus  and  two  plastids.  These  two 
plastids  divide  at  the  first  division  of  the  fertilized  egg  but  not  at  the 
second,  the  four  resulting  cells  consequently  having  one  plastid  each. 
In  the  third  cell-division  the  plastids  also  divide,  so  that  each  cell  of 
the  several-celled  structure  developing  from  the  fertilized  egg  has  its 
single  plastid.  Each  of  the  several  cells  eventually  becomes  a  zoospore 
which  germinates  to  produce  a  new  Coleochcete  body  with  a  single  plastid 
in  each  cell,  the  plastid  dividing  with  the  nucleus  at  each  cell-division. 
A  somewhat  similar  regularity  in  the  behavior  of  the  plastid  is  shown 
in  Anthoceros  (Davis  1899;  Scherrer    1911).     Each  gametophytic  cell 

8 


114 


INTRODUCTION  TO  CYTOLOGY 


contains  a  single  plastid  which  divides  with  the  nucleus  at  each  cell- 
division.  The  egg  likewise  contains  a  plastid,  but  the  spermatozoid  has 
none:  the  fertilized  egg  and  sporophyte  cells  which  it  later  forms  are 
therefore  characterized,  like  the  cells  of  the  gametophyte,  by  the  presence 
of  one  plastid.  Although  it  is  difficult  to  demonstrate  the  plastid  in  the 
young  sporogenous  cells,  every  sporocyte  shows  one  very  clearly.  As 
shown  by  Davis  (Fig.  42),  the  sporocyte  plastid  divides  twice  during  the 
prophases  of  the  first  (heterotypic)  division  of  the  sporocyte  nucleus,  so 
that  each  spore  of  the  resulting  tetrad  receives  one.  Upon  germination 
the  spore  produces  a  gametophyte  with  one  plastid  in  each  cell,  and  the 
cycle  is  complete. 


Fig.  42. — The  behavior  of  the  plastid  in  the  sporocyte  of  Anthoceros. 

A,  sporocyte  with  single  nucleus  and  plastid.  B,  plastid  divided;  nucleus  in  prophase 
of  mitosis.  C,  plastids  divided  to  four;  two  nuclei  present.  D,  three  of  the  four  spore 
cells,  each  of  which  has  a  single  nucleus  and  plastid.      (After  Davis,  1899.) 


In  all  of  the  foregoing  examples  it  is  evident  that  the  plastids,  as 
stated  by  Scherrer  for  Anthoceros,  remain  as  morphological  individuals 
throughout  the  whole  life  cycle,  multiplying  exclusively  by  division.  A 
similar  claim  is  made  for  the  plastids  of  mosses  by  Sapehin  (1915),  who 
has  also  studied  the  behavior  of  the  plastids  in  Selaginella  and  Isoetes 
(1911,  1913).  In  such  cases  the  plastids  each  possess  an  individuality  com- 
parable to  that  of  nuclei,  from  which  they  differ  conspicuously,  however, 
in  undergoing  no  fusion  at  the  time  of  fertilization.  The  constancy  in 
number  is  nevertheless  maintained :  by  the  degeneration  of  the  plastids  of 
one  gamete  in  Zygnema;  by  their  failure  to  divide  at  one  cell-division  in 
Coleochcete;  and  because  of  the  fact  that  the  male  gamete  carries  no 
plastid  in  Anthoceros.  It  appears  to  be  generally  true  that  while  the  eggs 
in  all  plant  groups  contain  plastids  (usualty  leucoplasts),  the  latter  are 
present  in  male  gametes  in  the  algae,  only.  Sapehin  (1913),  however, 
believes  that  the  blepharoplasts  of  the  higher  groups  represent  plastids. 

It  should  be  said  that  only  in  a  comparatively  few  forms  has  such  a 
regularity  in  the  behavior  of  the  plastid  as  that  outlined  above  been 
demonstrated.  A  number  of  investigators,  working  on  a  great  variety  of 
cells,  have  been  forced  to  conclude  that  plastids  are  either  formed  de  novo 
as  well  as  by  division,  or  are  carried  through  certain  stages_of  the  life 


PLASTIDS  AND  CHONDRIOSOMES  L15 

cycle  in  some  less  conspicuous  form.  If  they  represenl  regional  trans- 
formations of  the  cytoplasm  resulting  from  the  localization  of  certain 
processes,  they  might  well  be  expected  to  differentiate  anew  as  thi 

processes  begin  in  the  life  of  the  cell,  and  to  preserve  varying  degn 
of  permanence    depending    upon    the   processes   carried   on    (Harper). 
Their  individual  continuity  through  certain  life  cycles  would  accordingly 
be  interpreted  to  mean  that  in  such  forms  there  is  a  persistence  of  certain 
types  of  physiological  activity  through  all  stage-. 

In  recent  years  a  number  of  cytologists  have  described  the  develop- 
ment of  plastids  from  minute  granular  primordia  in  the  cytoplasm,  and 
have  attempted  to  show  that  these  primordia  are  members  of  the  class  of 
cell  inclusions  known  as  chondriosomes.  A  general  theory  of  the  indi- 
viduality of  the  plastid  must  therefore  involve  the  question  of  the  relation 
of  plastids  to  chondriosomes,  and  the  further  question  of  the  origin  of  the 
chondriosomes  themselves.  These  matters  will  be  taken  up  in  the  fol- 
lowing pages. 

CHONDRIOSOMES 

Notwithstanding  the  large  amount  of  work  which  has  been  done  upon 
chondriosomes  during  recent  years,  the  condition  of  opinion  as  to  their 
origin,  behavior,  and  significance  is  still  so  unsettled  that  little  more  than 
a  review  and  partial  classification  of  the  more  prominent  views  will 
here  be  attempted. 

Chondriosomes  were  probably  first  observed  many  years  ago  by 
Flemming  and  Altman  in  the  course  of  their  studies  on  protoplasm. 
They  were  first  clearly  described  by  La  Vallette  St.  George  (1886),  who 
observed  them  in  the  male  cells  of  animals  and  called  them  "cytomicro- 
somes."  In  plants  they  were  first  described  by  Meves  (1904)  in  the 
tapetal  cells  of  the  anthers  of  Nymphcea  (Fig.  43,  B).  Beinhi  in  L897 
and  the  following  years  discovered  them  in  cells  of  many  types,  notably 
in  the  spermatogenous  cells  of  animals,  and  applied  to  them  the  term 
"mitochondria."  It  was  not  until  a  decade  later,  through  the  researches 
of  Meves,  Regaud,  Faure-Fremiet,  Lewitski,  Guilliermond,  and  others 
that  they  came  into  prominence.  Since  that  time  they  have  been  very 
intensively  studied  by  both  zoologists  and  botanists,  and  a  Bpecial 
literature  of  considerable  bulk  has  developed.1  It  now  seems  evident 
that  the  filaments  ("fila")  of  Flemming,  the  "bioplasts"  of  Ah  man,  the 
" plastidules "  of  Maggi,  the  "archoplasmic  granules"  of  Boveri,  and  the 
"mitochondria"  of  Benda  are  all  one  and  the  same  1  hing  chondriosomes 
(Duesberg  1919). 

General  Nature  and  Occurrence.  -Chondriosomes  occur  in  the  cyto- 
plasm of  the  cell,  commonly  in  the  form  of  minute  granules,  rods,  and 

1  Reviews  of  the  subject  are  given  by  Duesberg  (1911,   L919),  Schmidt  (1912 

Cavers  (1914).  and  Guilliermond  (1919).     See  also  Meves  (1918  . 


116 


INTRODUCTION  TO  CYTOLOGY 


threads,  but  also  in  a  great  variety  of  irregular  shapes  (Fig.  43).  At 
present  it  is  customary  with  the  majority  of  workers  to  refer  to  all  types 
as  chondrio somes  or  mitochondria.  For  those  which  are  definitely  rod- 
and  thread-shaped  the  terms  chondriokonts  and  chondriomites  are  also 
used.  It  is  not  to  be  thought  that  the  various  forms  constitute  distinct 
classes,  for  several  investigators  (N.  H.  Cowdry;  M.  and  W.  Lewis  1915) 
have  observed  the  chondriosomes  undergoing  marked  changes  in  shape 
in  living  cells,  granular  ones  becoming  rod-shaped  and  filamentous,  and 
vice  versa.  Schaxel  (1911)  and  Kingery  (1917)  state,  moreover,  that  in 
fixed  material  the  shape  of  the  chondriosomes  is  to  a  certain  extent 
dependent  upon  the  character  of  the  microtechnical  methods  employed. 


3 


h 


V 


Ffc  43. — Chondriosomes  in  plant  and  animal  cells. 

A,  nerve  cell  from  guinea  pig.  X  480.  (After  E.  V.  Cowdry,  1914.)  B,  tapetal  cell 
of  Nymphoea  alba.  (After  Meres,  1904.)  C,  living  epidermal  cell  of  tulip  petal.  D,  ascus 
of  Pustularia  vesiculosa.  E,  hypha  of  Rhizopus  nigricans.  F,  portion  of  embryo  sac  of 
Lilium;  chondriosomes  clustered  about  nucleus.  G,  cell  of  root  tip  of  Allium — (C-F. 
After  Guilliermond,  1918.) 

Although  when  first  discovered  chondriosomes  were  believed  to  be 
rather  limited  in  distribution,  they  have  now  been  reported  in  the  cells  of 
plants  and  animals  belonging  to  nearly  all  of  the  larger  natural  groups. 
It  is  asserted  by  N.  H.  Cowdry  (1917)  that  "in  all  forms  of  animals, 
from  amoeba  to  man,  which  have  been  investigated  with  adequate 
methods  of  technique,  they  occur  without  exception."  They  are  present, 
furthermore,  in  the  cells  of  all  tissues.  In  plants  it  is  probable  that  they 
are  no  less  universally  present,  although  it  has  not  yet  been  possible  to 
demonstrate  them  with  certainty  in  bacteria,  Cyanophycese,  and  certain 
Chlorophycese,  such  as  the  Conjugate  and  Confer  vales  (Guilliermond 
1915).  They  are  abundant  in  myxomycetes  (N.  H.  Cowdry  1918), 
Charales  (Mirande  1919),  brown  and  red  algae,  fungi,  and  all  the  higher 
groups. 

A  critical  comparison  of  the  chondriosomes  of  plants  with  those  of 
animals  has  been  made  by  N.  H.  Cowdry  (1917),  who  concludes,  contrary 


PLASTIDS  AND  CHONDRIOSOMES  117 

to  the  opinion  of  Pensa  (1914),  that  there  is  every  reason  to  regard  them 
as  homologous  in  the  two  kingdoms.  He  finds  plant  and  animal  chon- 
driosomes  to  be  practically  identical  in  morphology,  reaction  to  fixatives 
and  dyes,  and  distribution  in  resting  and  dividing  cells:  any  conspicuous 
differences  in  arrangement  seem  to  be  due  to  the  more  pronounced 
polarity  of  the  animal  cell.  In  both  cases  they  are  most  abundant  in 
the  active  stages  in  the  life  of  the  cell.  As  the  cell  ages  and  becomes 
fully  differentiated,  i.e.,  as  cytomorphosis  proceeds,  they  diminish  in 
number  and  may  completely  disappear. 

Physico-chemical  Nature. — With  regard  to  the  chemical  and  physical 
nature   of  chondriosomes,   Regaud   (1908),   Faure-Fremiet    (1910).   and 
Lowschin  (1913),  working  respectively  on  mammals,  protozoa,  and  plants, 
agree  that  they  are  chemically  a  combination  of  phospholipin  and  albu- 
min.    They  closely  resemble  phosphatids,  which  are  combinations  of 
phosphoric  and  fatty  acids,   glycerol,  and  nitrogen  bases.     Lecithin  is 
such  a  compound.     Since  chondriosomes  are  soluble  in  alcohol,  ether. 
chloroform,  and  dilute  acetic  acid,  many  of  the  fixing  reagents  commonly 
employed  in  microtechnique  destroy  them:  this  accounts  in  part  for  the 
fact  that  they  were  not  observed  in  many  familiar  tissues  until  a  compara- 
tively recent  date.     They  are  well  fixed  by  neutral  formalin,  potassium 
bichromate,  osmium  tetroxid,  and  chromium  trioxid  (chromic  acid);  and 
these,  therefore,  are  the  principal  ingredients  of  the  fixing  reagents  em- 
ployed in  researches  upon  chondriosomes.     Examples  of  such  fluids  are 
those  of  Altman,  Benda,  Bensley,  Helley,  Kopsch,  Regaud,  and  Zenker.1 
Besides  staining  with  hematoxylin  and  several  other  dyes  commonly 
employed  with  fixed  material,  the  chondriosomes  show  a  characteristic 
affinity  for  certain  intra-vitam  stains,  such  as  Janus  green  B,  Janus  blue, 
Janus  black  I,  and  diethylsafranin,  the  reaction  with  the  first  of  these 
being  especially  strong.     After  certain  treatments  the  chondriosomes  may 
closely  resemble  the   "chromidial   substance,"   or  granules  of  nucleo- 
protein  distributed  throughout  the  cytoplasm  in  some  cells.     That  the 
two  are  not  to  be  confused  has  been  emphasized  by  Duesberg  and  by 
E.  V.  Cowdry.     According  to  the  latter  author  (1916)  chondriosomes  are 
"a  concrete  class  of  cell  granulations,"  and  may  be  provisionally  defined 
as  "substances  which  occur  in  the  form  of  granules,  rods  and  filaments  in 
almost  all  living  cells,  which  react  positively  to  Janus  green  and  which, 
by  their  solubilities  and  staining  reactions,  resemble  phospholipins  and 
to  a  lesser  extent,  albumins." 

Origin  and  Multiplication.- -The  questions  of  the  origin  and  multipli- 
cation  of    chondriosomes  are  much  debated  ones.      Certain   Cytologists 

1  For  convenient  summaries  of  the  effects  of  various  reagents  upon  chondriosomes 
the  student  may  refer  to  Kingsbury's  (1912)  paper  on  cytoplasmic  fixation,  E.  V. 
Cowdry's  (191-1)  on  vital  staining,  and  N.  H.  Cowdry's  (1917)  on  plant  and  animal 

chondriosomes. 


118 


ISTliODUCTION  TO  CYTOLOGY 


believe  that  they  have  found  good  evidence  for  the  view  that  chondrio- 
somes  may  multiply  by  division,  and  some  (Guilliermond;  Moreau  1914; 
Terni  1914)  have  held  this  to  be  their  sole  mode  of  origin— that  they  arise 
only  from  preexisting  chondriosomes  and  are  therefore  permanent  cell 
organs.  Others  are  convinced  that  they  may  arise  de  novo  in  the  cyto- 
plasm, and  that  the  evidence  for  their  division  is  unsatisfactory  (Orman 
1913;L6wschin  1913;  Scherrer  1914;  Miss  Beckwith  1914;  Chambers 
1915;  M.  and  W.  Lewis  1915;  Twiss  1919;  and  others).  The  investiga- 
tors of  the  foregoing  group,  together  with  Meves  (1900),  Lewitski  (1910), 
and  Forenbacher  (1911),  hold  that  the  chondriosomes  arise  from  the  cyto- 
plasm, but  certain  others  believe  they  take  their  origin  from  the  nucleus. 
Tischler  (1906)  and  Wassilief  (1907),  for  example,  state  that  they  arise 
from  surplus    chromatin.     AlexiefT   (1917)   thinks  that  although  cyto- 


0 


1  6 


Fig.  44. — Examples  of  regular  behavior  of  chondriosomes  in  cell-division. 

A-C,  spermatocyte  of  Oryllotalpa  vulgaris,  (After  Vo'inov,  1916):  A,  chondriosomal 
material  in  cytoplasm  about  nucleus;  B,  heterotypic  mitosis,  showing  chondriosomes  (at 
sides)  occupying  the  spindle  with  the  chromosomes  (at  center) ;  C,  stages  in  the  division 
of  a  chondriosome.  D,  Dividing  cell  of  Geotriton  fuscus,  showing  division  of  individual 
chondriosomes  as  cell  constricts  at  equator.      (After  Terni,  1914.) 

iplasmic  dfferentiation  is  due  to  them,  they  are  at  least  in  some  cases 
of  nuclear  origin;  and  further  that  they  are  not  fundamentally  different 
from  chromosomes  and  chromidia,  a  conclusion  contradictory  to  that  of 
Duesberg  and  E.  V.  Cowdry,  cited  above.  Shaffer  (1920)  believes  them 
to  arise  as  the  result  of  a  chemical  action  of  the  nucleus  upon  products  of 
assimilation  in  the  adjacent  cytoplasm.  Wildman  (1913)  classifies  the 
cytoplasmic  inclusions  present  throughout  spermatogenesis  in  Ascaris 
into  two  main  types,  both  of  nuclear  origin:  "karyochondria,"  equiva- 
lent to  the  mitochondria  of  other  writers,  and  "plastochondria,"  which 
pass  into  the  cytoplasm,  form  yolk  within  them,  and  fuse  to  form  the 
food  supply  (" refractive  body")  of  the  spermatozoon. 

That  the  behavior  of  the  chondriosomes  at  the  time  of  cell-division 
is  a  matter  of  considerable  importance  has  been  generally  recognized.  In 
many  cases  their  distribution  to  the  two  daughter  cells  seems  to  be  quite 
fortuitous,  whereas  in  some  tissues  more  or  less  definite  modes  of  distribu- 
tion have  been  described.  According  to  Faure-Fremiet  (1910),  Terni 
(1914),  Korotneff  (1909),  and  others,  the  individual  chondriosomes  divide 
at  the  lime  of  mitosis  (Fig.  44,  D),  a  conclusion  with  which  many  others 
fail  to  agree  (Orman  1913;  Miss  Beckwith  1914;  etc.).     In  the  cells  of 


PLASTIDS  AND  CH0NDRI0S0M1  119 

the  grasshopper,  Dissosteira  Carolina,  Chambers  (1915)  finds  thai  the 
chondriosomal  material  forms  a  granular  network  surrounding  the  nucleus 
during  the  resting  stages  and  the  mitol  ic  figure  during  division.  1  )uring 
the  later  phases  of  mitosis  the  strands  and  granules  of  tins  network 
lengthen  into  delicate  filaments  between  the  two  daughter  chromosome 
groups,  and  finally  separate  into  two  granular  masses  which  gradually 
invest  the  daughter  nuclei. 

In  the  mole  cricket,  Gryllotalpa  borealis,  the  distribution  of  the  chon- 
driosomes  to  the  daughter  cells  is  accomplished  with  even  greater  defin- 
iteness.  According  to  Payne  (1916)  they  become  thread-like  and  break- 
near  the  middle,  the  halves  passing  to  the  daughter  cells.  Voinov  I  1916) 
states  that  the  "mitochondria"  in  the  spermatocyte  of  G.  vulgaris  fuse  to 
form  a  thread  which  then  segments  into  a  number  (70  or  more)  "chondrio- 
somes." These  are  arranged  on  the  spindle  along  with  the  chromo- 
somes, which  they  may  closely  resemble,  and  divide  to  form  daughter 
bodies  at  both  maturation  divisions,  so  that  they  are  equally  distributed 
to  the  four  resulting  spermatozoa  (Fig.  44,  A-C). 

In  certain  scorpions  also  the  chondriosomal  material  is  distributed 
with  surprising  precision.  In  a  species  from  Arizona  (Wilson  191 6)  this 
material  in  the  spermatocyte  takes  the  form  of  a  single  ring-shaped  body. 
This  ring  divides  accurately,  much  like  a  chromosome,  at  both  maturation 
divisions,  each  of  the  four  spermatids,  and  hence  each  spermatozoon  of 
the  tetrad,  receives  a  quarter  of  its  substance.  In  a  California  species 
(Wilson)  there  is  no  ring  formed,  but  instead  about  24  hollow  spherical 
bodies.  At  the  two  maturation  divisions  these  show  no  evidence  of 
division,  but  are  passively  separated  into  four  approximately  equal  group-, 
each  spermatid  receiving  six  (occasionally  five  or  seven).  A  European 
species  described  by  Sokolow  (1913)  agrees  essentially  with  tin's. 

Function. — Our  knowledge  of  chondriosomes  is  yet  too  incomplete  to 
warrant  any  categorical  statements  regarding  their  functions,  but  a 
number  of  opinions  have- been  expressed,  some  of  them  based  upon  ob- 
servational evidence  and  others  upon  conjecture.  Certain  of  the  mote 
prominent  opinions  may  here  be  reviewed. 

It  was  in  1897  that  Benda  suggested  that  chondriosomes  might  be 
distinct  cell  organs  with  a  special  function.  In  a  series  of  papers  which 
began  to  appear  ten  years  later  Meves  (1907  etc.)  put  forth  and  empha- 
sized the  theory  that  they  play  an  important  role  in  heredity  that  they 
carry  the  hereditary  characters  of  the  cytoplasm.  Evidence  supporting 
this  view  was  seen  by  Meves  and  Benda  in  certain  experiments  of  God- 
lewski  which  seemed  to  show  that  the  appearance  of  certain  hereditary 
characters  is  dependent  upon  something  present  in  the  cytoplasm  rather 
than  in  the  nucleus.  (See  Chapter  XIV.)  This  theory  has  had  the 
support  of  a  number  of  investigators,  among  whom  are  the  botanists 
Cavers  (1914)  and  Mottier  (1916).     Voinov  (1916)  also  believes  that  the 


120 


INTRODUCTION  TO  CYTOLOGY 


regular  distribution  of  the  chondriosomal  substance  in  Gryllotalpa 
strongly  favors  the  view  that  this  substance  is  of  some  significance  in 
heredity.  It  is  probable,  however,  that  the  majority  of  cytologists  regard 
the  evidence  brought  forward  in  support  of  the  view  as  very  inadequate. 
Wildman  (1913)  points  out  that  the  chondriosomes  may  be  largely  lost 
during  spermatogenesis,  and  others  have  recalled  cases  in  which  the 
nucleus  is  the  only  portion  of  the  male  gamete  which  can  be  seen  to  enter 
the  egg  at  fertilization.  Meves  (1911,  1915)  and  Benda,  on  the  other 
hand,  show  that  chondriosomes  also  enter,  at  least  in  the  forms  studied 
by  them  (Fig.  45).  In  the  animal  spermatid  the  chondriosomes  appear 
most  commonly  to  contribute  to  the  formation  of  the  Nebenkern  of  the 

spermatozoon  (La  Vallette  St.  George  1886; 
Popoff  1907;  Chambers  1915;  Shaffer  1920;  and 
others),  in  some  cases  later  elongating  into  a 
sheath  around  the  axial  filament  of  the  tail 
(Shaffer  on  Cicada).  Duesberg  (1919)  states 
that  although  the  fate  of  the  chondriosomes  of 
the  spermatid  varies  in  different  animals,  they 
are  nevertheless  always  present  in  the  sperma- 
tozoon, and  that  it  has  not  been  clearly  shown 
in  any  case  that  they  do  not  enter  the  egg  at 
fertilization.  In  many  eggs  which  they  do  enter, 
however,  they  behave  with  great  irregularity 
during  the  subsequent  cleavage  stages  (Van  der 
Stricht,  etc.).     It  is  not  at  all  improbable  that 

Pig.  45. — r ertihzation  in  .        '  r 

FiiaHa  papulosa,  showing  they  are  in  some  way  concerned  in  the  reactions 
chondriosomes  of   sperma-    through     which    hereditary   characters   are   de- 

tozoon  (at  top)  distributing  ,  .  . 

themselves  in  the  cytoplasm  veloped  in  the  individual,  but  the  general 
?oiKhf  egg*     (After  Meves>    opinion    is  that  their  apparent  variability  and 

indefiniteness  in  behavior  in  so  many  cases  are 
against  the  view  that  they  take  any  part  in  the  transmission  of  factors 
upon  whose  presence  the  development  of  the  characters  depends 
(Gatenby  1918,  1919).  The  equal  distribution  of  chondriosomes  at 
the  time  of  cell-division  is  thought  to  be  without  any  significance  in 
this  connection  by  Harper  (1919). 

It  is  obvious  that  much  work  remains  to  be  done  before  the  possible 
relation  of  chondriosomes  to  heredity  and  development  can  be  made 
clear.  For  the  present  it  is  safest  to  assume,  as  will  be  emphasized  in 
later  chapters,  that  hereditary  transmission  is  the  function  of  the  nucleus, 
chiefly  if  not  entirely,  since  the  chromosomes  afford  a  mechanism  of 
precisely  the  kind  required  to  account  for  the  observed  distribution  of 
hereditary  characters. 

Meves  (1907a6,  1909)  and  Duesberg  (1909)  have  also  called  attention 
to  the  close  relation  of  chondriosomes  to  muscle  fibers  in  the  developing 


PLASTIDS  AND  CHONDRIOSOMES 


121 


chick  embryo.  They  believe  that  the  chondriosome  elongates  and 
directly  becomes  the  young  fiber.  Gaudissart  (1913),  on  ili«-  contrary, 
shows  that  the  fiber  does  not  arise  exclusively  from  the  chondriosome,  but 
that  the  primary  basis  is  furnished  by  the  plasmatic  reticulum  with 
which  the  chondriosomes  cooperate  in  building  up  the  fiber.  Although 
the  chondriosomes  thus  have  a  part  in  the  genesis  of  the  muscle  !':!;<  r. 
the  latter  is  not  a  "modified  filamentous  chondriosome,"  as  Duesberg 
believed. 

Hoven  (1910a)  and  Meves  have  similarly  attempted  to  show  that 
chondriosomes  are  concerned  in  the  differentiation  of  neurofibrils  and  the 
collagenous  fibers  of  cartilage.     Regaud   (1911),   Guilliermond    (1914  . 


c  j&m 


Fig.  46. 
A,  formation  of  fat  in  cell  of  rabbit  by  granular  and  rod-shaped  chondriosomes.  [From 
Guilliermond,  after  Dubreuil,  1913.)  B,  formation  of  needle-shaped  crystals  <>!"  carotin  in 
chromoplasts  derived  from  chondriosomes  in  epidermal  cell  of  Iris  petal.  {After  (mil; 
mond,  1918.)  C,  chondriosomes  and  chloroplasts  in  young  cell  of  Finns  banksiana.  X  750. 
(After  Mottier,  1918.)  D,  transformation  of  plastid  primordia  into  leucoplasts  in  i""t 
cell  of  Pisum;  some  of  the  leucoplasts  contain  starch.      (After  M<>tti(  r.) 


Hoven  (19106,  1911),  and  Lewitski  (1914)  have  thought  that  the  chon- 
driosomes may  in  some  cases  perform  a  secretory  function,  and  Dubreuil 
(1913)  has  associated  them  with  the  production  of  fat  (Fig.  li».  .1  ).  In 
the  oocyte  of  Cicada  Shaffer  (1920)  finds  them  transforming  into  yolk 
spherules.  The  activity  of  bodies  called  "plastochondria"  by  Wildman 
(1913)  in  the  elaboration  of  the  food  supply  in  the  spermatozoon  of 
Ascaris  has  already  been  mentioned. 

Relation  of  Chondriosomes  to  Plastids.-  (  meof  the  most  conspicuous 
views  regarding  the  significance  of  chondriosomes  is  thai  which  holds 
some  of  them  to  be  the  primordia  of  plastids.  After  studying  the  cells  of 
Pisum  and  Asparagus  Lewitski  (1910)  concluded  that  the  chondriosomes 
are  essential  constituents  of  the  cytoplasm,  and  that  they  develop  into 
chloroplasts  and  leucoplasts  in  the  cells  of  the  stem  and  root  respectively. 


122  INTRODUCTION  TO  CYTOLOGY 

Evidence  in  support  of  this  conception  was  contributed  by  Forenbacher 
(1911),  Pensa  (1914),  Cavers  (1914),  and  others.     Guilliermond  (1911- 

1920)  in  particular  was  led  by  the  results  of  his  extensive  researches  on 
the  subject  to  the  view  that  the  chondriosomes,  arising  only  from  preexist- 
ing ones  by  division,  persist  through  the  egg  and  embryonic  cells 
and  later  become  amyloplasts,  chloroplasts,  and  chromoplasts.  In 
this  he  saw  strong  evidence  for  the  individuality  of  the  plastid.  In 
1915  he  advanced  the  opinion  that  in  fungi  the  chondriosomes  function 
like  the  amyloplasts  of  higher  plants,  forming  reserve  products  as  the 
latter  form  starch.  In  this  development  of  chondriosomes  into  plastids 
Guilliermond  (1913-1915)  and  Moreau  (1914)  were  able  to  show  that  the 
chondriosomes  produce  within  them  certain  phenolic  compounds  which 
either  appear  at  once  as  anthocyanin  pigments,  or  as  colorless  products 
which  may  acquire  color  later  through  chemical  alteration  (Fig.  46,  B). 

Among  the  most  recent  researches  in  this  field  are  those  of  Mottier 
(1916,  1918)  on  the  cells  of  Zea,  Pisum,  Elodea,  Pinas,  Adiantum,  Antho- 
ceros,  Pallavicinia,'  Marchantia,  and  several  algae.  He  finds  that  leuco- 
plasts  and  chloroplasts  are  derived  from  small  rod-shaped  primordia 
(Fig.  46,  C,  D)  which  he  regards  as  permanent  cell  organs  of  the  same 
rank  as  the  nucleus.  Both  primordia  and  mature  chloroplasts  multiply 
by  fission.  In  the  cells  of  Marchantia,  Anthoceros,  and  the  seed  plants 
he  finds  also  a  second  series  of  bodies,  which  he  calls  chondriosomes: 
these  like  the  plastid  primordia,  are  permanent  cell  organs  multiplying 
by  division,  but  they  do  not  become  chloroplasts  or  leucoplasts.  Further- 
more, both  chondriosomes  and  primordia  are  thought  by  Mottier  to 
be  concerned  in  the  transmission  of  certain  hereditary  characters. 

It  is  also  reported  by  Emberger  (1920afr)  that  in  the  roots  and  spor- 
angia of  ferns  two  kinds  of  granular  elements  may  be  recognized  at  all 
times,  one  of  them  representing  the  initial  stage  of  plastid  development. 
Contrary  to  Mottier's  opinion,  however,  he  regards  both  kinds  as  true 
mitochondria.  Guilliermond  (1920)  likewise  distinguishes  two  such 
types  in  7m  germanica. 

P.  A.  and  P.  Dangeard  (1919,  1920),  as  a  result  of  their  researches 
on  the  cells  of  barley,  Selaginella,  Larix,  Taxus,  and  Ginkgo,  distinguish 
three  classes  of  cytoplasmic  structures  differing  in  reaction  to  reagents 
and  in  function.  In  their  initial  stages  all  have  the  granular  form.  The 
plastidomes  first  appear  as  minute  "mitoplasts,"  which  gradually  enlarge 
and  develop  into  plastids.  The  spheromes  are  at  first  recognizable  as 
"microsomes,"  some  of  which  may  be  seen  to  give  rise  to  fat  and  oil 
globules  while  others  appear  to  undergo  no  change.  The  vacuomes 
begin  their  history  as  "metachromes; "  these  elongate  and  form  a  peculiar 
network  which  later  develops  into  a  system  of  vacuoles.  Guilliermond 
(1920)  denies  the  metachromatic  nature  of  this  third  class  of  bodies,  and 
holds  them  to  be  quite  distinct  from  mitochondria. 


PLASTIDS  AND  CHONDRIOSOMES  L23 

The  existence  of  such  ;i  genetic  relationship  between  uhondriosome 
and  plastids  :is  that  described  above  has  been  denied  by  many  writers, 
among  whom  may  be  mentioned  Lundegardh  (1910),  Meyer  (1911  . 
Rudolph  (1912),  Lowschin  (1913,  1914),  Scherrer  (  L91  h.  Miss  Beckwith 
(1914),  Derschau  (1914),  von  Winiwarter  (1914),  Sapehin  (1915), 
Chambers  (1915),  M.  and  W.  Lewis  (1915),  and  Harper  L919).  These 
workers  for  the  most  part  hold  thai  chondriosomes  are  qoI  distind  cell 
organs  at  all,  but  regard  them  rather  as  more  or  less  transient  visible 
products  of  protoplasmic  activity.  Derschau  asserts  thai  they  arise 
neither  de  novo  nor  by  fission,  but  thai  they  are  merely  small  mass 
of  plastin  and  nuclein  concerned  in  nutrition,  arising  from  basichromatin 
at  the  surface  of  the  nucleus.  Miss  Beckwith  speaks  of  them  as  differ- 
entiation products  of  the  cytoplasm.  Lowschin,  who  made  some  ex- 
periments in  the  production  of  artificial  chondriosomes.  believes  them 
to  be  due  to  the  emulsified  state  of  the  protoplasm  and  in  some  instanci  - 
to  the  action  of  fixing  agents  upon  it.  To  Chamber-  they  appear  in 
living  cells  not  as  persistent  structures  but  as  temporary  physical  states 
of  the  colloidal  substances  composing  protoplasm.  M.  and  \Y.  Lewi- 
have  studied  them  in  tissue  cultures  and  observe  that  they  are  continually 
being  formed  and  used  up,  and  that  they  show  no  sharply  distinct  types. 
Faure-Fremiet  (1910a)  distinguishes  "mitochondria,"  which  have  an 
individuality  of  their  own  and  are  permanent  cell  organs,  from  'lipo- 
somes," which  are  temporary  accumulations  of  reserve  substance. 

The  almost  universal  occurrence  of  chondriosomes  in  the  cells  <»i 
living  organisms,  and  their  frequent  alterations  in  number  and  appear- 
ance, suggest  a  connection  with  some  fundamental  process  going  on 
almost  constantly  and  common  to  all  living  matter.  That  this  process 
may  be  oxidation,  the  chondriosomes  being  a  "structural  expression  of 
the  reducing  substances  concerned  in  cellular  respiration'  (Kingsbury 
has  been  regarded  as  highly  probable  by  Kingsbury  (1912),  Mayer, 
Rathery,  and  Schaeffer  (1914),  N.  H.  Cowdry  (1917,  L918),  and  others. 
Evidence  favoring  this  interpretation  is  seen  in  the  fact  that  the  chondrio- 
somes occur  so  widely  in  the  cytoplasm,  which  acts  as  a  reducing  sub- 
stance; and  also  in  the  close  similarity  bet  ween  their  chemical  composition 
and  that  of  phosphatids,  which  appear  to  be  capable  of  auto-oxidation. 

Conclusion. — From  the  foregoing  review  ii  should  be  more  than  plain 
that  the  state  of  our  knowledge  of  chondriosomes  is  such  thai  almost  do 
definite  final  statements  can  be  made  regarding  their  origin  and  function. 
The  evidence  at  hand  apparently  indicates  thai  the  class  of  cell  inclusions 
known  as  chondriosomes  comprises  a  variety  of  bodies  which  play  differ- 
ent roles  in  the  life  of  the  cell.  It  is  scarcely  open  to  doubt  thai  some  of 
them  are  temporary  accumulations  of  substances  involved  in  metabolism, 
appearing  and  disappearing  in  the  cell  in  a  manner  somewhal  analogous 
to  that  of  starch.     The  most  plausible  hypothesis  concerning  the  specific 


124  INTRODUCTION  TO  CYTOLOGY 

physiological  role  of  such  changeable  types  of  chondriosomes  is  that  they 
have  to  do  with  the  processes  of  oxidation  and  reduction — with  cellular 
respiration.  It  is  also  becoming  increasingly  apparent  that  other  chon- 
driosomes represent  the  juvenile  stages  in  the  development  of  plastids 
of  various  kinds,  and  that  they  are  in  some  way  concerned  in  the  forma- 
tion of  chlorophyll  and  other  pigments.  If  this  is  true  they  are  clearly 
of  the  highest  importance. 

Whether  or  not  any  of  the  chondriosomes  are  to  be  considered  as 
permanent  cell  organs  is  a  question  to  which,  in  view  of  the  conflicting 
testimony  of  competent  observers,  no  final  answer  can  at  present  be 
given.  To  determine  whether  these  minute  bodies  arise  de  novo  or 
always  multiply  by  division  is  a  matter  of  extreme  practical  difficulty. 
Until  this  question  is  settled  it  is  obviously  impossible  to  come  to  a  deci- 
sion regarding  the  individuality  of  those  plastids  which  appear  to  take 
their  origin  from  chondriosomes,  or  to  know  what  may  be  the  possible 
relation  of  chondriosomes  to  inheritance.  With  respect  to  the  latter 
point,  the  chondriosomes,  like  all  other  structures  concerned  in  meta- 
bolism, may  be  indirectly  associated  with  the  development  of  hereditary 
characters,  but  the  view  that  they  transmit  or  represent  differential 
factors  for  such  characters  is  as  yet  unsupported  by  adequate  evidence. 

From  the  fact  that  the  chondriosomes  may  not  preserve  their  indi- 
viduality at  all  times,  however,  it  does  not  follow  that  they  must  be  denied 
the  rank  of  cell  organs.  Their  great  variability,  indifferent  behavior  at 
the  time  of  cell-division  in  so  many  cases,  and  their  unknown  mode  of 
origin  are,  as  Kingsbury  (1912)  states,  against  the  view  that  they  are  cell 
organs;  and  it  is  doubtless  true  that  many  chondriosomes  should  for  such 
reasons  be  denied  such  rank.  On  the  other  hand,  those  chondriosomes 
which  seem  clearly  to  perform  important  and  specific  functions  in  the  life 
of  the  cell  should,  like  centrosomes  appearing  de  novo  at  each  cell-division, 
be  looked  upon  as  cell  organs,  though  not  as  permanent  ones  with  an 
uninterrupted  continuity. 

In  spite  of  the  fact  that  the  study  of  chondriosomes  has  so  far  raised 
more  problems  than  it  has  solved,  it  has  already  proved  of  much  value, 
for  it  has  turned  to  the  cytoplasm  some  of  the  attention  so  long  directed 
almost  exclusively  to  the  nucleus,  and  it  appears  that  many  problems  of 
much  importance  to  cytology  pertain  to  the  cytoplasm.  It  has  also 
been  of  great  service  in  bringing  about  a  closer  scrutiny  of  the  effects 
of  fixation  and  a  renewed  emphasis  upon  the  importance  of  the  study  of 
living  protoplasm.  Much  has  already  been  learned  as  the  result  of  this 
study,  but  the  solution  of  the  principal  problems  involving  chondriosomes 
must  await  the  results  of  further  research. 


PLASTIDS  AND  CHONDRIOSOMES  125 

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Ges.  23:  285-292.     pi.  13. 
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No.  2244,  pp.  2-3,  7.      (Transl.  from  Rev.  Gen.  Sci.    Paris.) 
Beckwith,  C.  J.     1914.     The  genesis  of  the  plasma  structure  m  the  egg  of  Hydrac- 

tinia  echinata.     Jour.  Morph.  25:   189-251.     pis.  8. 
Beer,  R.     1909.     On  elaioplasts.     Ann.  Bot.  23 :  63-72.     pi.  4. 
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1898.     Ueber  die  Sperm atogenese  der  Vertebraten  und  hoherer  Evertebraten.     2. 

Die   Histogenese  der  Spermien.     Ibid,    1898. 
1902.     Die  Mitochondria.     Ergeb.  d.  Anat.  u.  Entw.  12 :  743-781.      (Review.) 
Binz,    A.     1892.     Beitrage   zur    Morphologic   und   Entwicklung   der    Starkekorner. 

Flora  76:  34-91.     pis.  5-7. 
Bourquin,  H.  1917.     Starch  formation  in  Zygnema.     Bot.  Gaz.  64 :  426-434.     pi.  27. 
Carter,    N.     1919a6.     Studies  on  the  chloroplasts  of  desm ids.     I,  II.     Ann.  Bot.  S3: 
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1919c.     The  cytology  of  the  Cladophoracea?.     Ibid.  33:  467-478.     pi.  27. 
1920a.    Studies  on  the  chloroplasts  of  desmids.     III.     The  chloroplasts  of  Cos- 

marium.     Ann.  Bot.  34:  265-285.     pis.  10-13. 
19206.     Studies  on  the  chloroplasts  of  desmids.     IV.  Ibid.  34 :  303-320.     pis.  1  1    16. 
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Phytol.  13 :  96-106.     (Review  of  subject.) 
Chambers,  R.     1915.     Microdissection  studies  on  the  germ  cell.     Science  41:  290- 

293. 
Chmielewskij.     1896.     Ueber  Bau  und    Vermehrung   der   Pyrenoide    bei    einigen. 

Algen.     (Russian,  10  pp.)     See  Bot.  Centr.  69:  277-278.     1897. 
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Ag.    Ann.  Bot.  33 :  323-352.     pis.  22-24.     figs.  3. 
Cowdry,  E.  V.      1914a.     The  vital  staining  of  mitochondria  with  Janus  green  and 
diethylsafranin  in  human  blood  cells.     Intermit.  Monatsschr.  f.  Anat.  u.  Physiol. 
31:  267-286. 
19146.     The  comparative  distribution  of  mitochondria  in  spinal  ganglion  cells  ol 

vertebrates.     Am.  Jour.  Anat.  17:  1-29.     pis.  3. 
1916.     The  general  functional  significance  of  mitochondria.      Am.  Jour,  Anat.  19: 

423-446.     (Review  of  subject.) 
Cowdry,  N.  H.     1917.     A  comparison  of  mitochondria  in  plan)  and  animal  cells. 

Biol.  Bull.  33:  196-228.     figs.  26. 
1918.     The  cytology  of  the  myxomycetes  with  Bpecial  reference  u>  mitochondria. 

Biol.  Bull.  35 :  71-94.     1  pi. 
1920.      Experimental  studies  on   mitochondria  in   plant  cells.      Ibid.  39:    188   206 

pis.  3. 
Dangeard,  P.  A.     1919.     Sur  la  distinction  du  chondriome  des  auteurs  en  vacuome, 
plastidome,  et  spherome.     Compt.   Rend.  Acad.  Sci.  Paris  169:   1005   1010. 


126  INTRODUCTION  TO  CYTOLOGY 

1920a.     Plastidome,    vacuome   e1    sph6rome   dans  Selaginella   Kraussiana.     Ibid. 

170:  301-306.      1  pi. 
19206.     La  structure  de  la  cellule  vegetale  el  son  metabolisme.     Ibid.  170:  70!) 

714. 
Daxgeard,  P.      1020.     Sur  revolution  du  systeme  vacuolaire  chez  les  gymnospermes. 

Ibid.  170.  474-477.     figs.  8. 
Davis,  B.  M.     1899.     The  spore  mother  cell  of  Anthoceros.     Bot,  Gaz.  28:  89-109. 

pis.     9,  10. 
von  Derschau,    M.     1914,  Zum  Chromatindualismus  der  Pflanzenzelle.     Arch.  f. 

Zellf .  12 :  220-240.     pi.  77. 
Dodel,    A.      1892.     Beitrag    zur    Morphologie    und  Entwicklung  der  Starkekorner 

von  Pellionia  Daveauana.     Flora  75:  267-2S0.     pi.   5-6. 
Dubreuil,  G.     1913.     Le  chondriome  et  le  dispositif  de  l'activite  secretaire.     Arch. 

d'Anat.  Micr.  15:  53-151. 
Duesberg,  J.     1907.     Der  Mitochondrialapparat  in  den  Zellen  der  Wirbeltierc  und 

Wirbellosen.     Arch.  Mikr.  Anat,  71 :  284-296.     pi.  24. 
1909.     Les  chondriosomes  des  cellules  embryonnaires  du  poulet  et  leur  role  dans  la 

gencse  des  myofibrilles,  avec  quelques  observations  sur  le  developpement  des 

fibres  musculaires  striees.     Arch.  Zellf.  4 :  602-671.     pis.  28-30.     figs.  10. 
1911.     Plastosomes,     "apparato     reticolaro     interno,"     und     Chromidialapparat. 

Ergeb.  d.  Anat.  u.  Entw.  20:  567-916.     (Extensive  review.) 

1917.  Chondriosomes  in  the  cells  of  fish  embryos.     Am.  Jour.  Anat.  21:  465-493. 
pis.  1-3.     figs.  8. 

1918.  Chondriosomes  in  the  testicle-cells  of  Fundulus.     Am.    Jour.    Anat.   23: 
133-154.     pis.  2.     figs.  21. 

1919.  On  the  present  status  of  the  chondriosome-problem.     Biol.  Bull.  36  :  71-81. 
Duesberg,  J.  et  Hoven,  H.     1910.     Observations  sur  la  structure  du  protoplasme 

des  cellules  vegetales.     Anat.  Anz.  B  36 :  96-100.     figs.  5. 
Emberger,  L.     1920a.     Evolution  du  chondriome  chez  les  cryptogames  vasculaires. 

Compt.  Rend.  Acad.  Sci.  Paris  170:  282-284.     figs.  5. 
19206.     Evolution  du  chondriome  dans  la  formation  du  sporange  chez  les  fougeres. 

Ibid.  170  :  469-471.     figs.  7. 
Faure-Fremiet,    M.    E.     1910a.     Mitochondries    et    liposomes.     Comptes   Rend. 

Soc.  Biol.  Paris  62 :  537-539. 
19106.     La  continuite  des  mitochondries  a  travers  des  generations  cellulaires  et 

le  role  de  ces  elements.     Anat.  Anz.  36:  186-191.     figs.  3. 
Fischer,  A.     1905.     Die  Zelle  der  Cyanophyceen.     Bot.  Zeit.  63:  51-129.     pis.  4,  5. 
Forenbacher,    A.     1911.     Die   Chondriosomen  als  Chromatophorenbildner.     Ber. 

Deu.  Bot.  Ges.  29 :  648-660.     pi.  25. 
Franze,  R.     1893.     Zur  Morphologie  und  Physiologie  der  Stigmata  der  Mastigo- 

phoren.     Zeit.  Wiss.  Zool.  56:  138-164.     pi.  8. 
Gardner,  N.  L.     1906.     Cytological  studies  in  Cyanophycese.     Univ.  Calif.  Publ. 

Bot.  2:  237-296.     pis.  21-26. 
Gargeanne,    A.    J.    M.     1903.     Die    Oelkorper    der    Jungermanniales.     Flora    92 : 

457-482.     figs.  18. 
Gatenby,  J.  B.     1918.     Cytoplasmic  inclusions  of  the  germ  cells.     III.  The  sper- 
matogenesis of  some  other  Pulmonates.     Quar.  Jour.   Micr.  Sci.  63 .  197-258. 

pis.  16-18.     figs.  3. 
1919.     The  cytoplasmic  inclusions  of  the  germ  cells.     V.  The  gametogenesis  and 

early  development  of  Limncea  stagnalis  (L.)  with  special  reference  to  the  Golgi 

apparatus  and  the  mitochondria.     Quar.   Jour.   Micr.   Sci.   63:  445-492.     pis. 

27,  28.     figs.  6.     (See  also  Parts  I  and  II  in  vol.  62,  and  Part  IV  in  63.) 


PLASTIDS  AND  CHONDRIOSOMES  127 

Gaudissabt,  P.     1913.     Reseau  protoplasmique  et  chondriosomea  dans  la  genese  des 

myofibrilles.     La  Cellule  30:  29-43.     pis.  1,  2. 
Gottsche.     1843.     Anatomische-physiologische  Untersuchangen  iiber  Haplomitrium 

Hookeri.     Verh.  Leop.  Carol.  Akad.  12:  I,  286. 
Guignard,    L.     1889.     Developpement    et    constitution    des    antherozoides.      Rev. 

Gen.  Bot.  1;  11,  63,  136,  175.     pis.  2-6. 
Guilliermond,  A.     1911.     Sur  Lea  mitochondries  des  cellules  veg&ales.     Compl 

Rend.  Acad.  Sci.  Paris  153:  109-201.     figs.  4. 

1912.  Recherches  cytologiques  sur  le  mode  de  formation  de  I'amidon  et  sur  lea 
plastes  vegetaux.     Arch.  d'Anat.  Micr.  14 :  309-428.     pis.  13   18. 

1913.  (a)  Sur  la  signification  du  chromatophore  des  algues.     Comp.  Rend.  8 
Biol.  Paris  75:  85-87.     (b)  Quelques  remarques  nouvelles  sur  la  formation  dea 
pigments    anthocyaniques    au    sein    des    mitochondries.     Ibid.    47s    181. 
Xouvelles  observations  sur  le  choudriome  de  Fasque  de  Pustvlaria  vesiculosa. 
Ibid.  646-649.     (d)  Xouvelles  remarques  sur  la  signification  des  plastes  de  \\ 
Schimper  par  rapport  aux  mitochondries  actuelles.     Ibid.  437-440. 

1914a.     Etat  actuel  de  la  question  de  revolution  et  du  role  physiologique  des 

mitochondries.     Rev.  Gen.  Bot.  26:  129-149,  182-210.     figs.  16. 
19146.     Bemerkungen  iiber  die  Mitochondrien  der  vegetativen  Zellen  und  ihre 

Verwandlung  in  Plastiden.     Ber.  Deu.  Bot.  Ges.  32:  282-301.     figs.  2. 
1915a.     Nouvelles  observations  vitales  sur  le  chondriome  des  cellules  epidermiquea 

de  la  fleur  d'lris  germanica.     Comp.  Rend.  Soc.  Biol.  Paris  67:  241-249. 
19156.     Recherches  sur  le  chondriome  chez  les  champignons  et  les  algues.     Rev. 

Gen.  Bot,  27:  193,  236,  271,  297,  315.     pis.  12. 
1917a.     Sur  la  nature  et  le  role  des  mitochondries  des  cellules  vegetales.     Comp. 

Rend.  Soc.  Biol.  Paris  69:  916-924. 
19176.     Observations  vitales  sur  le  chondriome  de  la  fleur  de   Tulipe.     Comp. 

Rend.  Acad.  Sci.  Paris  164:  407-409. 
1917c.     Contributions  a  l'etude  de  la  fixation  du  cytoplasme.     Ibid.  643  646. 
1917a\     Recherches  sur  l'origine  des  chromoplastes  et  le  mode  de  formation  de 

pigments  du  groupe  des  xanthophylles  et  des  carotins.     Ibid.  232-234. 
1917e.     Sur  les  alterations  et  les  caracteres  du  chondriome  dans  les  cellules  epi- 

dermique  de  la  fleur  de  Tulipe.     Ibid.  609-612. 
1917/.     Sur  les  phenomenes  cytologiques   de  la  degenerescence   de   cellules   epi- 

dermiques  pendant  la  fanaison  des  fleurs.     Compt.  Rend.  Soc.  Biol.   Paris  69: 

726-729. 

1918.  Sur  l'origine  mitochondriale  des  plastids.     Compt.  Rend.  Acad.  Sri.  Paris 
167 :  430-433. 

1919.  Observations  vitales  sur  le  chondriome  des   vegdtaux   et    recherches  but 
l'origine   des   chromoplastides  et  le  mode  de  formation  de-  pigments  xantho- 
phylliens  et  carotiniens.     Rev.  Gen.  Bot.  31:  372    113.  446  508,  •">•;•-,  603,  »;.;:, 
770.     pis.  60.     figs.  35. 

1920a.     Sur  revolution  du  chondriome  dans  la  cellule  veg6tale.     Compt.    Rend. 
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19206.     Sur  les  elements  figures  du  cytoplasme.      11. id.  170:  612  til"-,      figs.  5. 

1920c.     Nouvelles  recherches  sur  l'apparei]  vacuolaire  dans  les  v.'g.'tauv      [bid. 
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1905.     Ueber  die    Plasmahaut  der  Chloroplasten   in  den    Assimilationzellen  von 
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128  INTRODUCTION  TO  CYTOLOGY 

Harper,  R.  A.     1919.     The  structure  of  protoplasm.     Am.  Jour.  Bot.  6  :  273-300. 
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Jahrb.  Wiss.  Bot.  36:  229-354.     pis.  5,  6.     figs.  5. 
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19106.     Contribution  a  l'etude  du  fonctionnement  des  cellules  glandulaires.     Du 

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Tricladen.     Arch.  Mikr.  Anat.  74:  1000-1016.     pis.  2. 
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pi.  11. 
Kurssanow,  L.     1911.     Ueber  Befruchtung,   Reifung  und  Keimung  bei  Zygnema. 

Flora  104 :  65-84.     pis.  1-4. 
KtisTER,  W.     1894.     Die  Oelkorper  der  Lebermoose  und  ihr  Verhaltniss  zu  Elaio- 

plasten.     Inaug.  Dissert.,  Basel. 
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Ber.  Deu.  Bot.  Ges.  29:  362-369.     figs.  4. 
La  Valette  St.    George,    A.   1886.     Spermatologische    Beitrage.  2.     Arch.  Mikr. 

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Lewitski,  G.     1910.     Ueber  die  Chondriosomen  in  pflanzlichen  Zellen.     Ber.  Deu. 

Bot.  Ges.  28:  538-546.     pi.  17. 
1911.     Die   Chloroplastenanlagen   in   lebenden    und   fixierten   Zellen   von   Elodea 

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1  pi. 
Lidforss,  B.     1893.     Studien  ofver  elaiosferer  i  ortbladens  mesophyll  och  epidermis. 

Inaug.  Dissert.,  Lund;  also  in  Kgl.  Fysiogr.  Sallsk.  i  Lund  Handl.  4. 


PLASTIDS  AND  CHONDRIOSOMES  129 

Lowschin,  A.  M.     1913.     "Myelinformen"  und  Chondriosomen.     Ber.   Deu.   Bot. 
Ges.  31:  203-209. 
1914.     Vergleiehende     experimental-cytologiache     Unterauchungen     Qber     Mito- 
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pi.  5. 
Lundegardh,    H.     1910.     Kin   Beitrag   zur    Kritik   zweier    Vererbungsliypotheseii. 

Jahrb.  Wiss.  Bot.  48:  285-378. 
Mast,  S.  O.     1911.     Light  and  the  behavior  of  organisms,     pp.  410.     N.  V. 

1916.     The    process    of    orientation  in  the  colonial  organism,  G&nium  pedoraXe^ 
and  a  study  of  the  structure  and  function  of  the  eyespot.     Jour.  Exp.  Zool.  20. 
1-17.     figs.  6. 
Mayer,    Rathery    and    Schaeffer.     1914.     Les    granulations    ou    mitochondrial 

de  la  cellule  hepatique.     Jour.  Physiol.  Path.  Gen.  16:  007-022. 
McAllister,    F.     1913.     Nuclear  division   in    Tetraspora  lubrica.     Ann.    Bot.    27: 
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1914.     The  pyrenoid  of  Anthoceros.     Am.  Jour.  Bot.  1:  79-95.     pi.  8. 
Meves,    Fr.     1904.     Ueber  das  Vorkommen  von  Mitochondrien  bezw.     Chondrio- 
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1907a.     Ueber  Mitochondrien  bzw.     Chondriokonten  in  den  Zellen  junger  Embryo- 

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19076.     Die    Chondriokonten    in   ihrem    Verhaltnis    zur    Filarmasse    Flemmings. 
Ibid.  31:  561-569. 

1908.  Die  Chondriosomen  als  Trager  erblicher  Anlagen.     Cytologische  StudieD 
am  Hiihnerembryo.     Arch.  Mikr.  Anat.  72:  816-867.     pis.  39-42. 

1909.  Ueber  Neubildung  quergestreifter  Muskelfasern  nach  Beobachtungen  am 
Hiihnerembryo.     Anat.  Anz.  34:  161-165.     figs.  3. 

1914.  Was  sind  die  Plastosomen?     Ibid.  85:  279-302.     figs.  17. 

1915.  Ueber   Mitwirkung  der   Plastosomen  bei  der  Befruchtung  dea   Eiea  von 
Filaria  papillosa.     Arch.  Mikr.  Anat.  87:  II  12-46.     pis.  1-4.     See  also  p.  286. 

1918.     Die  Plastosomentheorie  der  Vererbung.     Eine  Antwort  auf  verschiedener 
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18836.     Das  Chlorophyllkorn.     pp.  91.     pis.  3.     Leipzig. 
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1911.     Bemerkungen   zu    G.    Lewitski:  Ueber   die  Chondriosomen  in  pflanzlichen 
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Mirande,  M.     1919.     Sur  le  chondriome,  les  chloroplastes  ct  Lea  corpuaculea  Ducleo- 
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von    Mohl,    H.    1835,    1837.     Ueber    die  Vermehrung   der    Pflanzensellen   durch 
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Moreau,   F.     1914.     Le  chondriosome  et  la  division   dea   mitochondriea  chei   lea 

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Mottier,   D.   M.     1918.     Chondriosofnes   and  the  primordia  of  chloroplasta  and 
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9 


L30  INTRODUCTION  TO  CYTOLOGY 

von  Nageli,  C.  1858.     Die  Starkekorner.     Zurich. 

Olive,  E.  W.     1904.     Mitotic  division  of  the  nuclei  of  the  Cyanophycese.     Beih.  Bot. 

Centr.  18:  9-44.     pis.  1,  2. 
Orman,    E.     1913.     Recherches    sur   les   differenciations   cytoplasmiques   dans   les 

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1-4. 
Overton,  E.     1889.     Beitrag  zur  Kenntniss  der  Gattung  Volvox.     Bot.  Centr.  39: 

65-72.     113-1  IS,   145-150,   177-182,  209-214,  241-246,  273-279.     pis.  4. 
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Payne,  F.     1916.     A  study  of  the  germ  cells  of  Gryllotalpa  borealis  and  Gryllotalpa 

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Soc.  Biol.     Paris  65:  660-662. 

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(Review.) 
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Bot,  32:  117-166.     pis.  1,  2. 
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319-398.     pis.  10-26. 
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1-56.     pis.  1-3. 
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pi.  2. 


PLASTIDS  AND  CHONDRIOSOMES  131 

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421-457.     pis.  3 


132  INTRODUCTION  TO  CYTOLOGY 

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1894.     Sammel-Referate.     9.   Die     Chromatophoren.     10.   Die     Augenfleck.     11. 

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verwandten  Korper.     Beih.  Bot,  Centr.  4:  90-101,  161-165,  165-169,  329-335. 


CHAPTER  VII 
METAPLASM ;  POLARITY 

In  the  foregoing  chapters  we  have  described  successively  the  various 
organs  of  the  cell.  Our  account  of  the  resting  cell  will  now  be  com- 
pleted by  passing  in  brief  review  some  of  its  more  conspicuous  non- 
protoplasmic  inclusions.  We  shall  also  call  attention  to  another 
characteristic  but  imperfectly  understood  attribute  of  the  protoplast, 
namely,  its  polarity. 

Metaplasm. — In  addition  to  their  definite  cell  organs— nucleus, 
cytoplasm,  centrosomes,  plastids,  and  possibly  chondriosomes  -cells 
which  have  undergone  any  amount  of  differentiation  usually  contain  a 
variety  of  other  materials  representing  products  of  metabolism.  Many 
of  these  substances  are  held  in  solution  in  the  cell  sap,  itself  a  differentia- 
tion product,  while  others  are  present  in  insoluble  form  in  the  cytoplasm, 
often  in  special  vacuoles.  All  such  non-protoplasmic  inclusions,  partic- 
ularly those  existing  in  some  visible  form,  are  referred  to  as  metaplasm, 
a  term  introduced  by  Hanstein.  Although  it  has  been  held  by  sonic 
(Kassowitz  1899)  that  metaplasm  is  always  inactive  and  to  be  sharply 
set  apart  from  active  protoplasm,  it  is  more  probable,  as  Child  (1915 
contends,  that  no  absolute  distinction  can  be  made  between  the  two. 
Most  of  the  products  of  differentiation,  however,  are  clearly  non-proto- 
plasmic and  relatively  inactive. 

In  cells  of  many  types,  even  in  the  comparatively  undifferentiated 
cells  of  the  root  meristem,  there  often  occur  accumulations  of  chemically 
complex  substances  in  the  form  of  small  globules  or  irregular  masses  in 
the  cytoplasm.  In  many  cases  these  more  or  less  transient  bodies,  which 
often  stain  intensely  with  the  nuclear  dyes  and  arc  therefore  referred  to 
as  "chromatic  bodies,"  show  reactions  indicating  a  composition  closely 
approaching  that  of  the  extra-nuclear  granules  of  uucleo-protein  (chro- 
midia)  which  R.  Hertwig  and  Goldschmidt  interprel  as  granules  of  escaped 
chromatin  concerned  in  cell  differentiation.  Others  resemble  the  fatty 
chondriosomes  in  form  and  composition.  It  is  therefore  a  mat  ter  of  some 
difficulty  to  distinguish  between  these  various  substances,  which,  as  a 
matter  of  fact,  probably  do  not  represent  sharply  distinct  classes. 

The  most  conspicuous  non-protoplasmic  inclusions  represent  food 
materials  in  transitory  form  or  in  the  storage  condition;  they  are  conse- 
quently abundant  in  cells  carrying  a  supply  of  reserve  foods,  such  as 
spores  and  eggs,  and  in  storage  organs,  such  as  many  roots  and  the  endo- 

1 33 


134 


INTRODUCTION  TO  CYTOLOGY 


sperm  and  cotyledons  of  seeds.  In  the  animal  egg  the  storage  material 
commonly  exists  in  the  form  of  "  yolk  globules,"  or  "deutoplasm  spheres," 
which  consist  for  the  most  part  of  relatively  complex  protein  compounds. 
Fat  or  oil  globules  are  usually  present  with  them.  In  plants  the  most 
characteristic  storage  product  is  starch,  the  origin  and  characters  of  which 
were  described  in  Chapter  VI  along  with  the  plastids  by  which  they  are 
formed.  In  some  organisms,  including  the  fungi,  glycogen  appears  to 
carry  on  the  function  performed  by  starch  and  sugar  in  the  higher  plants. 
Fats  and  oils,  usually  in  the  form  of  droplets  but  sometimes  of  soft  grains 
or  even  crystals  (nutmeg),  comprise  another  important  class  of  storage 
substances :  these  are  especially  prevalent  in  seeds  and  spores,  where  light 
weight  is  of  advantage.  In  many  cases  oil  may  be  produced  anywhere  in 
the  cell,  but  in  certain  forms  it  has  been  found  that  special  elaioplasts,  and 


f  & 

Fig.  47. — Crystalline  and  other  inclusions  in  the  cells  of  various  plants. 
A,  cystolith  in  subepidermal  cell  of  Ficus  leaf.  B,  crystal  cells  in  Arctostaphylos.  C, 
druse  in  cell  of  Rheum  palmatum.  D-K,  aleurone  grains:  D,  E,  from  Myristica;  F,  from 
Datura  stramonium;  G,  from  Ricinus  communis;  H,  from  Amygdalus  communis;  I,  from 
Bcrtholletia  excelsa;  J,  from  Fceniculum;  K,  from  Elceis  guiniensis.  L,  raphides  in  leaf 
of  Agave.     M,  inulin  crystals  in  preserved  cells  of  artichoke.      (B-K  after  Tschirch.) 


possibly  also  chondriosomes,  are  concerned  in  this  process.  The  peculiar 
oil  bodies  found  in  the  cells  of  certain  liverworts  appear  to  represent  oil 
vacuoles:  these  also  have  been  discussed,  together  with  elaioplasts,  in 
Chapter  VI.  Large  masses  of  intranuclear  metaplasm  are  found  charac- 
teristically in  the  eggs  of  gymnosperms. 

Aleurone  grains  occur  in  small  vacuoles  in  the  cells  of  many  seeds, 
particularly  in  such  oily  ones  as  those  of  Ricinus,  Juglans,  and  Bertholl- 
etia.  In  maize  and  wheat  grains  they  are  limited  to  a  single  layer  of  cells, 
the  " aleurone  layer."  The  aleurone  grain  varies  much  in  structure  and 
form,  several  types  being  described  by  Pfeffer  in  1872.  The  grain  con- 
sists primarily  of  an  amorphous  protein  substance,  often  with  an  outer, 
somewhat  more  opaque  shell.  Some  examples  show  no  greater  differen- 
tiation than  this,  but  many  are  much  more  elaborate  (Fig.  47,  D-K). 
Those  of  Ricinus  contain  within  them  a  single  angular  crystal  of  protein 
(albumen),  often  referred  to  as  the  "  crystalloid,"  and  a  globule  of  a  double 
phosphate  of  calcium  and  magnesium  with  certain  organic   substances 


METAPLASM;  POLARITY  L35 

called  the  "globoid,JI  (Fig.  47,  (?)■  The  crystalline  inclusions  sometimes 
grow  to  l)c  very  Large.  K  has  been  thoughl  by  certain  workers  that 
aleurone  grains  arc  self-perpetuating  bodies  with  an  individuality  com- 
parable lo  that  of  nuclei  and  certain  plastids.     That  this  view  is  correct 

has  been  rendered  very  improbable  by  the  researches  of  East  and  I  Lives 
(1911,  1915)  and  Emerson  (1914,  1917)  on  the  inheritance  of  aleurone 
characters  in  maize,  and  also  by  the  work  of  Thompson  (1912),  who  suc- 
ceeded in  producing;  artificial  aleurone  grains  in  all  essential  respects 
similar  to  those  elaborated  by  the  plant. 

Crystals  occur  in  great  variety  in  the  differentiated  cells  of  plants. 
They  may  lie  in  the  cytoplasm,  in  vacuoles,  attached  to  or  imbedded  in 
the  cell  wall,  and  even  in  special  cells.  They  are  usually  salts  of  calcium, 
calcium  oxalate  being  especially  prevalent.  The  bundles  of  needle- 
shaped  crystals  known  as  "raphides"  (Fig.  47,  L)  found  in  the  leaves  of  a 
number  of  plants  are  composed  of  the  latter  salt,  as  are  also  the  spherical 
aggregations  called  " druses,"  or  "sphserraphides"  (Fig.  47,  C).  The 
curious  clustered  "cystoliths"  of  the  Ficus  leaf  (Fig.  47,  A)  are  made  up  of 
cellulose  and  calcium  carbonate.  Crystals  of  silica  are  very  abundant  in 
the  thickened  walls  of  wood  cells  and  in  many  other  tissues,  such  as  the 
outer  cells  of  the  Equisetum  stem.  Crystals  of  albumen,  aside  from  those 
found  in  aleurone  grains,  are  frequently  present  in  the  cytoplasm  of  cells 
poor  in  starch,  as  in  the  outer  portion  of  the  potato  tuber.  The  leucoplast 
of  Phajus  often  contains  a  rod-shaped  albumen  crystal.  Protein  crystals 
of  various  shapes  are  occasionally  observed  within  the  nucleus  (Stock 
1892;  Zimmermann  1893). 

Cellulose  is  a  common  storage  material,  existing  as  a  rule  in  the  form 
of  laminae  deposited  upon  the  original  cell  wall. 

As  already  pointed  out,  the  sap  of  vacuolated  cells  may  contain  a 
number  of  differentiation  products  in  solution.  The  cell  sap  is  usually 
slightly  acid  in  reaction,  owing  to  the  presence  or  organic  acids  (malic. 
formic,  acetic,  oxalic)  and  their  salts.  Inogranic  salts  are  probably  always 
present.  Amides,  such  as  glutamin  and  asparagin,  glucosides,  sugars, 
proteins,  tannin,  and  many  other  substances  are  of  frequent  occurrence 
in  the  cell  sap  of  various  plants.  The  carbohydrate  inulin  max  be  pre- 
cipitated out  of  the  sap  by  alcohol:  this  accounts  for  the  presence  of 
nodules  of  radiating  inulin  crystals  frequently  encountered  in  preserved 
material  (Fig.  47,  M).  Rubber  is  present  in  the  form  of  a  suspension  of 
minute  droplets  in  the  cell  sap  of  Ficus  elastica  and  several  other  plant-. 
Gutta-percha  occurs  in  a  similar  state  in  Tsonandra  gutta.  The  cell  sap 
in  such  cases  has  a  characteristic  milky  appearance.  The  cell  sap  is 
often  colored  by  red,  blue,  and  yellow  anthocyanin  pigments  Wheldale 
1916;  Palladin  1918;  Beauverie  1919),  some  of  which  change  color  when 
the  reaction  of  the  sap  is  altered  from  acid  to  basic  and  via  versa.  The 
striking  colors  of  flowers  are  due  to  "(1)  the  varying  color  of  the  sap,  (2) 


136  INTRODUCTION  TO  CYTOLOGY 

the  distribution  of  the  cells  containing  it,  and  (3)  combinations  of  colored 
sap  with  chloro-  and  chromoplasts."  Autumnal  coloring  is  due  to  the 
formation  of  pigments  as  disorganization  products:  when  cytoplasm  and 
chlorophyll  are  the  main  disorganizing  substances  a  yellowish  color  results, 
whereas  if  sugars  are  present  in  considerable  amounts  in  the  cell  sap  the 
brighter  pigments  are  formed. 

Extruded  Chromatin.1 — The  actual  extrusion  of  chromatin  from  the 
nucleus  into  the  cytoplasm  has  been  reported  in  a  number  of  instances: 
in  the  microsporocytes  of  various  angiosperms  by  Digby  (1909,  1911, 
1914),  Derschau  (1908,  1914),  West  and  Lechmere  (1915),  and  others; 
in  ferns  by  Farmer  and  Digby  (1910);  and  in  the  Ascomycete  Helvella 
crispa  by  Carruthers  (1911).  The  extruded  chromatin  commonly  takes 
the  form  of  deeply  staining  globules  or  irregular  masses  in  the  cytoplasm; 
often  a  clear  area  suggesting  a  nuclear  vesicle  is  present  about  them.  In 
some  cases,  such  as  Gallonia  candicans  (Digby  1909)  and  Lilium  candidum 
(West  and  Lechmere  1915),  the  chromatin  may  pass  through  the  wall 
into  an  adjacent  cell,  where  it  forms  a  rounded  mass  connected  by  a 
chromatic  strand  with  the  nucleus  from  which  it  originated. 

The  significance  of  this  phenomenon  is  by  no  means  apparent.  It  is 
not  at  all  unlikely  that  nutritive  materials  passing  from  nucleus  to  cyto- 
plasm during  the  normal  metabolism  of  the  cell  occur  at  times  as  visible 
globules  at  the  nuclear  surface.  The  extrusion  of  chromatin  into  neigh- 
boring cells,  on  the  other  hand,  in  many  cases  has  every  appearance  of 
a  phenomenon  associated  with  degeneration  or  some  other  abnormal 
physiological  condition.  West  and  Lechmere,  however,  view  the  process 
as  one  which  occurs  normally  at  certain  stages,  and  which  will  probably 
be  found  to  be  more  general  in  plants.  Sakamura's  (1920)  extensive 
researches  on  chloralized  cells  have  led  him  to  regard  the  extrusion  of 
large  masses  of  chromatin  as  an  abnormal  phenomenon  which  occurs  as  a 
result  of  a  disturbance  of  the  metabolism  of  the  cell.  Its  more  frequent 
occurrence  in  sporocytes  than  in  other  cells  is  attributed  to  the  unusual 
sensitiveness  of  the  former  to  disturbing  influences. 

The  Senescence  of  the  Cell. — The  accumulation  of  products  of 
metabolism  ("differentiation  products")  has  a  direct  bearing  on  the 
problem  of  protoplasmic  senility.  As  its  life  progresses  the  cell  gradually 
"ages, "  and  if  nothing  occurs  to  prevent  it  the  process  eventually  term- 
inates in  death.  What  shall  be  taken  as  an  index  of  the  degree  of  senes- 
cence has  been  the  subject  of  much  discussion.  We  have  already  called 
attention  to  the  attempts  which  have  been  made  to  correlate  senescence 
with  a  progressive  change  in  the  nucleoplasmic  relation,  concluding 
that  no  constant  correlation  of  the  kind  has  been  shown  to  exist  (p.  63). 

Child  (1915)  has  brought  forward  much  evidence  to  show  that  the 

1  Extruded  chromatin  is  not  metaplasm,  but  it  has  been  found  convenient  to 
treat  it  at  this  point  along  with  other  inclusions  of  the  cytoplasm. 


METAPLASM;  POLARITY  137 

relative  rate  of  metabolism  is  the  main  criterion  of  the  cell's  physiological 

age,  "young"  cells  having  a  high  rate  and  "old'  cells  a  relatively  low 
rate,  and  a  gradual  decline  in  this  rate  occurring  throughout  the  life  of 
the  cell.  In  embryonic  (physiologically  young)  cells  the  cytoplasm  ap- 
pears to  be  comparatively  homogeneous  and  undifferentiated.  Older 
cells,  on  the  contrary,  are  ordinarily  marked  by  the  presence  of  products 
of  differentiation  in  the  cytoplasm.  The  true  measure  of  age  is  there- 
fore not  time,  but  physiological  differentiation. 

In  many  cells  a  rejuvenating  process  may  occur,  whereby  a  high  meta- 
bolic rate  is  restored  and  the  products  of  differentiation  lost:  this  is 
regarded  as  a  "return  to  the  embryonic  state" — a  real  physiological 
rejuvenescence.  "Senescence  is  primarily  a  decrease  in  rate  of  the 
dynamic  processes  conditioned  by  the  accumulation,  differentiation,  and 
other  associated  changes  of  the  material  of  the  colloid  substratum. 
Rejuvenescence  is  an  increase  in  rate  of  dynamic  processes  conditioned 
by  changes  in  the  colloid  substratum  in  reduction  and  dedififerentiation  " 
(Child,  p.  58).  Such  a  rejuvenescence  occurs  in  connection  with 
regeneration,  vegetative  and  other  asexual  reproduction,  and  sexual 
reproduction.  In  each  case  the  cell  which  begins  the  new  life  cycl< — the 
meristematic  regenerating  cell,  the  zoospore,  or  the  zygote — has  a  high 
metabolic  rate  and  is  comparatively  free  from  the  products  of  differentia- 
tion. 

In  the  lower  organisms  cell  differentiation  in  this  sense  is  not  so  great 
but  that  almost  any  cell  may  retain  the  power  to  "dedifferentiate"  and 
begin  the  development  of  a  new  individual  vegetatively.  In  these  forms 
asexual  reproduction  may  occur  repeatedly  and  keep  the  organism  as  a 
whole  (in  protozoa  and  protophyta)  or  the  protoplasm  of  the  race  (in 
lower  metazoa  and  metaphyta)  physiologically  young.  Only  when  the 
metabolic  rate  falls  very  low  does  sexual  reproduction,  the  most  effective 
of  all  the  rejuvenating  agencies,  ensue. 

In  the  higher  plants  the  retention  of  the  power  of  dedifferentiatinn 
is  strikingly  shown  in  the  well  known  cases  of  Begonia  and  BryophyUum, 
which  can  regenerate  complete  new  individuals  from  a  few  leaf  cells. 
In  the  higher  animals  cell  differentiation  is  usually  so  great  thai  the 
somatic  cells  can  no  longer  dedifferentiate  and  reproduce  the  organism 
asexually.  Here  rejuvenation  occurs  only  after  the  union  of  two  gametes, 
which  are  themselves,  unlike  the  zoospores  of  algae,  physiologically  old. 
Although  local  rejuvenescence  may  occur,  as  in  secretory  cells  which  are 
"younger"  after  secretion,  and  also  in  wound  tissue,  the  differentiation 
of  the  body  cells  is  carried  so  far  that  their  metabolic  rate  falls  low  enough 
to  make  a  recovery  or  rejuvenescence  no  longer  possible.  Tim-  ii  i- 
only  the  functioning  reproductive  ('('lis  that  endure:  the  ultimate  cessation 
of  all  life  processes  in  the  body  cells  is  the  price  which  is  inevitably  paid 
by  the  complex  multicellular  organism  for  the  advantages  conferred  by 
its  high  degree  of  differentiation. 


138  INTRODUCTION  TO  CYTOLOGY 

Of  the  highest  importance  in  this  connection  are  the  results  of  at- 
tempts to  maintain  the  cells  and  tissues  of  higher  animals  in  the  living 
condition  in  artificial  culture  media  outside  the  body.  It  has  been  shown 
by  the  remarkable  experiments  of  Carrel,  Leo  Loeb,  Burrows,  H.  V. 
Wilson  and  others  that  cells  may  be  isolated  from  any  of  the  highly 
differentiated  essential  tissues  of  the  body  and  kept  actively  growing  and 
multiplying  in  vitro  for  a  length  of  time  frequently  far  exceeding  that  to 
which  they  would  have  lived  in  the  body.  They  do  not  appear  to  grow 
old:  indeed  it  is  not  improbable  that  in  such  a  constantly  favorable 
environment  somatic  cells  are  as  " potentially  immortal"  as  the  germ 
cells  (see  p.  403).  In  the  words  of  Pearl  (1921),  "It  is  the  differentiation 
and  specialization  of  function  of  the  mutually  dependent  aggregate  of 
cells  and  tissues  which  constitutes  the  metazoan  body  which  brings  about 
death,  and  not  any  inherent  or  inevitable  mortal  process  in  the  indivi- 
dual cells  themselves." 

POLARITY 

Polarity  is  a  feature  which  is  exhibited  in  some  form  by  the  cells  of 
all  higher  organisms,  and  in  at  least  many  of  the  simpler  ones,  as  shown  by 
Tobler  (1902,  1904)  for  certain  algae;  indeed  it  is  probable  that  it  is 
possessed  in  some  -form  and  degree  by  all  cells.  Harper  (1919)  calls 
attention  to  the  fact  that  "in  the  presence  of  polarity  and  the  various 
symmetry  relations  we  have  a  fundamental  distinction  between  cell 
organization  and  that  of  polyphase  colloidal  systems  as  they  are  com- 
monly produced  in  vitro." 

This  polarity  has  two  aspects,  the  morphological  and  the  physiological. 
In  the  first  place,  the  various  constituents  of  the  cell  may  be  arranged 
symmetrically  about  one  or  more  ideal  axes,  so  that  the  cell  has  more 
or  less  distinctly  differentiated  anterior  and  posterior  ends.  This 
structural  aspect  of  polarity  has  been  the  one  chiefly  emphasized  by 
certain  workers:  van  Beneden  (1883),  for  instance,  looked  upon  polarity 
as  "a  primary  morphological  attribute  of  the  cell,"  the  axis  passing 
through  the  nucleus  and  the  centrosome.  Later  writers,  among  them 
Heidenhain  (1894,  1895),  made  this  conception  of  morphological  polarity 
the  basis  for  interpretations  of  many  of  the  phenomena  of  cell  behavior. 
(See  Wilson  1900,  pp.  55-56.)  However,  as  Harper  (1919)  points  out, 
polarity  "is  apparently  independent  of  the  uni-  or  multinucleated  condi- 
tion of  the  cell,  which  shows  that  it  is  in  some  cases  at  least  a  more 
generalized  characteristic  of  the  cell  as  a  whole  rather  than  a  mere  ex- 
pression of  the  space  relations  of  the  nucleus  and  cytoplasm  ..."  Other 
investigators  (Hatschek  1888;  Rabl  1889,  1892)  early  laid  emphasis  upon 
the  physiological  expression  of  polarity.  The  cell  shows  a  polar  differ- 
entiation in  physiological  labor:  the  processes  in  one  portion  of  the  cell 
differ  from  those  in  another,  this  difference  in  the  case  of  tissue  cells 


METAPLASM;  POLARITY  \:\\\ 

being  due  to  different  environments  in  the  tissue.     For  these  work* 
this  physiological  differentiation   is   the  essential  element    of  polarity; 
any  morphological  polarity  is  due  secondarily  to  it. 

Metabolic  Gradient.— The  most  suggestive  physiological  conception 

recently  developed  in  this  connection  is  that  of  Child  (1911  1916). 
Child  has  shown  in  the  case  of  Planaria  and  other  lower  animals,  as  well 
as  in  certain  algae,  that  along  each  of  the  axes  of  symmetry  there  exist-  a 
"metabolic  gradient,"  or  " axial  gradient :"  the  rate  of  the  physiological 
processes  is  highest  at  one  end  of  the  axis  and  diminishes  progressively 
toward  the  other  end.-  The  anterior  end  of  a  planarian,  for  example, 
has  a  higher  metabolic  rate  than  the  posterior  portion-.  Furthermore, 
the  portions  of  higher  rate  dominate  and  control  the  development  of 
those  portions  having  a  lower  rate,  with  the  result  that  the  young  indivi- 
dual soon  develops  and  maintains  a  definite  physiological  correlation  of 
anterior  and  posterior  parts.  Similarly  in  individuals  with  more  t  han  one 
axis  of  symmetry,  there  may  be  a  corresponding  dorsal-ventral,  as  well 
as  an  axial-marginal,  correlation.  That  polarity  is  here  primarily  a 
physiological  matter  is  indicated  by  the  fact  that  experimental  altera- 
tions in  the  metabolic  rate  in  different  parts  is  followed  by  abnormalities 
in  structural  development. 

As  to  the  means  by  which  the  dominance  of  certain  regions  over  others 
is  exercised,  correlating  the  activities  of  the  various  parts  of  the  or- 
ganism, there  are  two  principal  theories  in  the  field.  According  to  one 
theory  chemical  substances  (hormones)  are  produced  at  certain  places 
and  transmitted  through  the  body.  Although  the  circulation  of  such 
hormones  clearly  has  much  to  do  with  correlation  in  higher  complex 
organisms,  Child  adduces  good  evidence  in  support  of  the  second  theory, 
namely,  that  the  fundamental  relations  of  polarity  "depend  primarily 
upon  impulses  or  changes  of  some  sort  transmitted  from  the  dominant 
region,  rather  than  upon  the  transportation  of  chemical  substances1 
(p.  224). 

It  cannot  at  present  be  said  to  what  extent  this  conception  of  polarity 
is  applicable  to  the  single  cell.  The  work  of  Child  shows  in  a  very 
definite  manner  the  coincidence  of  the  morphological  and  physiological 
axes  of  polarity,  which  indicates  that  the  two  are  but  different  aspects  of 
one  and  the  same  polar  differentiation.  A  similar  coincidence  exists  very 
generally  in  the  case  of  the  single  cell.  In  the  cell,  as  in  t  he  organism  as  a 
whole,  functional  and  structural  differential  ion  are  inseparably  connected. 
In  the  present  state  of  our  knowledge  the  attempt  to  determine  the  real 
essence  of  polarity  raises  questions  which  cannot  ye1  be  answered.  D< 
physiological  polarity  depend  upon  a  polarized  structure  which  is  a 
fundamental  attribute  of  the  cell's  ultimate  organization?  Or  does  a 
polarized  morphological  arrangement  follow  and  depend  upon  a  physio- 
logical division  of  labor  arising  as  a  difference  in  intensity  or  rate  in  proc- 


140  INTRODUCTION  TO  CYTOLOGY 

esses  originally  common  to  all  parts  of  the  cell?  If  so,  to  what  internal 
or  external  factors  is  the  establishment  of  this  difference  due  in  cells  having 
no  initial  polarity?  Analogies  with  electrical  polarity  have  been  resorted 
to  in  this  connection,  concerning  which  Harper  (1919)  says:  "To  pro- 
vide an  adequate  basis  for  understanding  the  observed  facts  of  polarity, 
however,  it  seems  to  me  that  the  conception  of  compound  aggregate 
polyphase  systems  is  more  suggestive  than  these  attempted  analogies  .  .  . 
In  the  spatial  arrangement  and  interactions  of  these  systems  polar  dif- 
ferences of  the  most  diversified  types  are  bound  to  arise  in  the  mass  as  a 
whole  and  express  themselves  in  the  form  and  relative  rigidity  and  surface 
tension  of  different  parts,  as  well  as  in  the  interrelations  between  the  cells 
of  a  group  in  contact." 

The  polarity  of  the  multicellular  organism  as  a  whole  is  closely  bound 
up  with  the  polarities  of  its  constituent  cells.  Harper  has  clearly  shown 
(1918)  that  in  Pediastrum  the  position  of  the  swarm-spores  in  the  colony 
which  they  unite  to  form  is  directly  dependent  upon  their  polarity. 
This  does  not  mean,  however,  that  the  polarity  of  the  multicellular  organ- 
ism is  nothing  more  than  the  sum  'of  the  polarities  of  its  constituent 
cells,  unless  we  return  to  Schwann's  simple  conception  of  the  organism  as 
merely  an  aggregate  of  independent  cells.  (See  p.  12.)  The  higher 
individuality,  the  colony,  has  its  own  polarity,  which  may  be  related  to, 
but  is  not  the  same  as,  that  of  its  individual  cells.  In  the  ordinary  multi- 
cellular organism  the  polarity  is  an  outgrowth  of  the  polarity  of  the 
fertilized  egg  cell  rather  than  of  the  polarities  of  the  many  adult  tissue 

cells. 

In  polarity,  then,  we  encounter  another  problem  which  must  be 
brought  nearer  a  solution  before  we  can  have  any  adequate  understanding 
of  the  relation  of  the  cell  to  the  multicellular  organism  as  a  whole,  and  of 
the  perplexing  matter  of  organic  individuality. 

Bibliography  7 
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METAPLASM;  POLARITY  1  11 

1913.  Studies,  etc.     VI.     The  nature  of  the  axial  gradients  in  Planana  and  their 
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unci  venvandten  Korper.     Beih.  Bot.  Centr.  4:  105-109,  321-335. 


CHAPTER  VIII 

SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY 

SOMATIC  MITOSIS 

Since  the  time  when  the  cell  was  pointed  out  as  the  unit  of  structure 
and  function  it  has  been  recognized  that  the  mode  of  origin  of  new  cells 
is  a  matter  of  fundamental  importance.  We  have  seen  in  our  historical 
sketch  that  cells  were  believed  by  the  founders  of  the  Cell  Theory  to 
arise  de  novo  from  a  mother  liquor,  or  "cytoblastoma,"  a  misconception 
removed  by  later  investigations  in  which  it  was  shown  beyond  question 
that  cells  arise  only  by  the  division  of  preexisting  cells.  By  several  early 
observers  the  nucleus  was  seen  to  have  a  more  or  less  prominent  part  in 
the  process,  its  division  preceding  that  of  the  cell,  but  "it  was  not  until 
1873  that  the  way  was  opened  for  a  better  understanding  of  the  matter. 
In  this  year  the  discoveries  of  Anton  Schneider,  quickly  followed  by 
others  in  the  same  direction  by  Butschli,  Fol,  Strasburger,  Van  Beneden, 
Flemming,  and  Hertwig,  showed  cell-division  to  be  a  far  more  elaborate 
process  than  had  been  supposed,  and  to  involve  a  complicated  trans- 
formation of  the  nucleus  to  which  Schleicher  (1878)  afterward  gave  the 
name  karyokinesis.  It  soon  appeared,  however,  that  this  mode  of  divi- 
sion was  not  of  universal  occurrence;  and  that  cell-division  is  of  two  widely 
different  types,  which  Van  Beneden  (1876)  distinguished  as  fragmenta- 
tion, corresponding  nearly  to  the  simple  process  described  by  Remak, 
and  division,  involving  the  more  complicated  process  of  karyokinesis. 
Three  years  later  Flemming  (1879)  proposed  to  substitute  for  the» 
terms  direct  and  indirect  division,  which  are  still  used.  Still  later  i  ISVJ 
the  same  author  suggested  the  terms  mitosis  (indirect  or  karyokinetic 
division)  and  amitosis  (direct  or  akinetic  division),  which  have  rapidly 
made  their  way  into  general  use,  though  the  earlier  term-  arc  often 
employed.  Modern  research  has  demonstrated  the  fact  that  amitosis 
or  direct  division,  regarded  by  Remak  and  his  followers  as  of  universal 
occurrence,  is  in  reality  a  rare  and  exceptional  process;.  .  .  it  is  certain 
that  in  all  the  higher  and  in  many  of  the  lower  forms  of  life,  indirect 
division  or  mitosis  is  the  typical  mode  of  cell-division"'  Wilson  1900, 
pp.  64-65). 1 

'The  following  additional   historical  data   are  of   interest.     The  chroraoson 
though  they  appeared  in  the  figures  of  Schneider    L873),  were  first  adequately  drawn 
by  Strasburger  in  1875.     Longitudinal  splitting  was  described  by  Flemming  in  18$ 
The  terms  prophase,  metaphase,  ;m<l  anaphase  were  introduced  by  Strasburger  in 

I  13 


144 


INTRODUCTION  TO  CYTOLOGY 


In  view  of  the  fact  that  the  phenomena  of  growth,  differentiation, 
reproduction,  and  inheritance  are  now  known  to  be  intimately  bound  up 
with  the  process  of  cell-division,  it  is  obvious  that  a  detailed  knowledge 
of  this  process  is  an  absolute  prerequisite  to  a  solution  of  many  of  the 
problems  which  confront  us.  In  the  present  chapter  the  essential  fea- 
tures of  vegetative  or  somatic  nuclear  division  will  be  described.  After  a 
preliminary  sketch  of  the  process  of  mitosis  we  shall  take  up  in  some 
detail  the  behavior  of  the  chromosomes  and  the  question  of  their  individ- 
uality. In  the  following  chapter  attention  will  be  devoted  to  other 
features  of  cell- division:  the  achromatic  figure,  the  mechanism  of  mitosis, 
cytokinesis  (the  division  of  the  extra-nuclear  portion  of  the  cell),  and  the 
formation  of  the  cell  wall. 


SOMATIC    MITOSIS 


PROPHASE} 


METAPHASE 


TELOPHASE!    s 


Fig.  48. — Diagram  of  a  typical  case  of  somatic  mitosis  in  plants. 


Preliminary  Sketch  of  Mitosis. — The  main  steps  in  a  typical  case  of 
somatic  mitosis  in  plants  may  be  very  briefly  outlined  as  follows  (Fig. 
48): 

The  chromatic  material  of  the  "  resting ?;  nucleus,  as  described  in 
Chapter  IV,  exists  in  the  form  of  a  more  or  less  irregular  reticulum.  As 
the  process  of  mitosis  begins  this  reticulum  resolves  itself  into  a  definite 
number   of  slender   threads   which   represent    chromosomes.     These 


m 


1884,  and  Heidenhain  in  1894  first  used  the  term  telokinesis  (telophase).  Lundegardh 
(19126)  added  interphase.  The  chromosome  was  named  by  Waldeyer  in  1888. 
Hermann  in  1891  distinguished  connecting  fibers  (central  spindle)  and  mantle  fibers. 
That  the  halves  of  each  split  chromosome  go  to  opposite  poles  was  shown  by  van 
Beneden  for  animals  and  by  Heuser  for  plants  in  1884.  The  achromatic  spindle  was 
first  figured  by  Kowalevsky  (1871)  and  Fol  (1873),  and  first  carefully  described  by 
Blitschli  (1875afe). 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY        145 

many  nuclei  are  distinct  from  each  other  from  the  first,  whereas  in  other 
cases  they  may  be  arranged  end-to-end  in  a  more  or  less  continuous 
thread,  or  spireme,  which  later  segments  transversely  into  independent 
chromosomes.  The  slender  threads  (chromosomes)  now  split  longitu- 
dinally throughout  their  entire  length.  A  progressive  shortening  and 
thickening  of  the  split  threads  ensues,  so  that  the  nucleus  is  eventually 
seen  to  have  a  certain  number  of  chromosomes  which  have  become 
double  through  a  longitudinal  cleavage. 

While  the  above  changes  are  occurring,  fine  fibrils  are  differentiated 
in  the  cytoplasm  near  the  nucleus  and  become  arranged  in  two  opposed 
groups.  The  nuclear  membrane  now  disappears  and  the  fibers  extend 
into  the  nuclear  region,  where  some  of  them  (the  "mantle  fibers") 
attach  themselves  to  the  double  chromosomes,  while  others  (the  ''con- 
necting fibers")  pass  through  from  one  pole  to  the  other.  The  double 
chromosomes  quickly  become  arranged  in  a  single  plane  at  the  equator 
of  the  cell,  the  fibers  meanwhile  forming  the  achromatic  figure,  or  spindle. 
This  stage  is  known  as  the  metaphase;  all  the  steps  leading  up  to  it,  be- 
ginning with  the  initial  changes  in  the  resting  reticulum,  constitute  the 
prophase. 

The  daughter  chromosomes  (the  halves  of  the  longitudinally  split 
chromosomes)  now  move  apart  toward  the  poles  of  the  achromatic 
figure,  where  they  soon  form  two  closely  packed  groups  with  the  central 
spindle  of  connecting  fibers  extending  between  them.  The  period  during 
which  the  daughter  chromosomes  are  thus  moving  apart  is  known  as  the 
anaphase.  The  two  groups  of  daughter  chromosomes  now  reorganize 
the  daughter  nuclei,  in  each  of  which  the  chromosomes  again  form  a 
reticulum  like  that  of  the  original  mother  nucleus.  This  reorganization 
period  is  called  the  telophase.  During  the  telophase  there  is  formed 
upon  the  connecting  fibers  (central  spindle)  a  separating  wall,  which 
completes  the  division  of  the  cell.  The  nucleolus  as  a  rule  plays  no  con- 
spicuous part  in  mitosis:  it  usually  disappears  during  the  late  stages  of 
the  prophase,  new  nucleoli  being  formed  in  the  daughter  nuclei  in  the 
telophase.  In  rapidly  dividing  cells  the  period  between  two  successive 
mitoses  is  called  the  interphase. 

Mitosis  in  animals  (Fig.  49)  is  closely  similar  to  that  in  plants  ;i- 
regards  the  behavior  of  the  chromosomes.  It  normally  differs  in  two 
conspicuous  features,  namely,  the  presence  of  centrosomes  and  the 
mode  of  cytokinesis  following  the  division  of  the  nucleus. 

During  the  prophases  the  centrosome  with  its  aster,  it"  not  already 

double,   divides.     The   two   daughter  centrosomes,   each    with    its    own 

aster,  move  apart,  and  a  small  bundle  of  fibers  extends  between  them: 

all  these  structures  together  form  the  amphiaster.      The  pays  on  the  side 

toward  the  nucleus  extend  into  the  latter  when  the  membrane  dissolves 

and  become  attached  to  the  chromosomes,  often  before  the  two  centro- 
10 


146 


INTRODUCTION  TO  CYTOLOGY 


somes  have  reached  polar  positions.  The  centrosomes,  surrounded  by 
asters,  remain  at  the  poles  of  the  achromatic  figure  during  metaphase, 
anaphase,  and  telophase,  and  after  mitosis  has  been  completed  they  may 
disappear  or  remain  through  the  resting  stage  to  function  in  the  next 
mitosis.     (See  p.  78.) 

The  division  of  the  cell  following  nuclear  division  is  commonly 
brought  about  in  animals  by  the  formation  of  a  cleavage  furrow,  which 
grows  inward  from  the  periphery  as  described  in  the  following  chapter, 
rather  than  by  the  formation  of  a  wall  on  the  spindle  fibers  as  in 
plants. 


Fig.  49. — Diagram  of  a  typical  case  of  somatic  mitosis  in  animals. 


Although  the  two  above  points  serve  in  general  to  distinguish  mitosis 
in  animals  from  that  in  plants,  the  distinction  is  not  a  sharp  one:  cen- 
trosomes  are  regularly  present  in  the  cells  of  many  lower  plants,  while 
cytokinesis  by  furrowing  also  occurs  in  certain  cases,  as  will  later  be 
shown.  The  essential  point  to  be  borne  in  mind  is  that  the  significant 
feature  of  mitosis — the  division  of  the  chromatin  and  its  distribution  to 
the  daughter  nuclei — is  fundamentally  the  same  in  both  plants  and 
animals. 

The  relative  duration  of  the  various  phases  of  mitosis  has  been  studied 
in  a  few  cases.  As  an  example  may  be  taken  the  observations  of  M.  and 
W.  Lewis  (1917)  on  the  mesenchyme  cells  of  the  chick  growing  in  tissue 
cultures.  These  investigators  summarize  the  researches  of  others  upon 
the  subject  and  give  the  following  figures  for  the  chick  cells:  prophase, 
5  to  50  minutes,  usually  more  than  30;  metaphase,  1  to  15,  usually  2 
to  10;  anaphase  1  to  5,  usually  2  to  3;  telophase  up  to  cytokinesis,  2  to  13, 
usually  3  to  6;  telophasic  reconstruction  of  daughter  nuclei,  30  to  120; 
total,  70  to  180  minutes. 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY       1  17 

Detailed  Description  of  the  Behavior  of  the  Chromosomes  in  Somatic 
Mitosis.1 — In  the  present  account  we  shall  depart  from  the  order  usually 
followed  in  descriptions  of  mitosis.  Instead  of  commencing  with  the 
resting  nucleus  and  tracing  the  steps  leading  to  the  formal  ion  of  two 
daughter  resting  nuclei,  we  shall  begin  the  description  with  the  fully 
formed  chromosomes  as  they  appear  at  the  metaphase  and  follow  them 
through  anaphase,  telophase,  resting  stage,  and  prophase  to  the  nexl 
metaphase,  when  they  are  again  clearly  seen.  This  is  done  in  order  thai 
the  account  of  the  telophasic  transformation  of  the  chromosomes  to 
form  a  resting  reticulum,  and  the  prophasic  condensation  of  the  Latter 
to  form  chromosomes,  may  be  given  without  interruption,  which  -('(•in- 
advisable in  view  of  the  nature  of  certain  questions  which  are  later  to 
be  discussed  in  the  light  of  chromosome  behavior. 

Metaphase  (Fig.  50,  A). — As  the  chromosomes  arrange  themselves 
upon  the  spindle  preparatory  to  their  anaphasic  separation  their  double 
nature  is  clearly  evident.  The  two  halves  may  lie  very  close  together 
and  in  the  case  of  long  chromosomes  may  be  somewhat  twisted  about 
each  other.  When  they  lie  a  little  apart  they  may  often  show  small 
connecting  strands  or  anastomoses;  in  the  immediately  preceding  stages 
(late  prophase)  the  halves  are  usually  pressed  tightly  together,  so  thai 
these  anastomoses  appear  to  be  due  to  mutual  coherence  at  certain 
points  when  the  halves  move  slightly  apart  after  the  disappearance  of 
the  nuclear  membrane.  As  the  double  chromosomes  take  their  places 
on  the  spindle,  the  spindle  fibers  become  attached  to  them,  not  to  all  parts 
but  to  a  particular  portion  of  each.  In  the  case  of  long  chromosomes 
the  point  of  attachment  is  often  at  about  the  middle,  whereas  in  shorter 
ones  it  is  commonly  near  one  end.  At  their  points  of  attachmenl  to 
the  spindle  the  double  chromosomes  all  lie  with  their  halves  superposed 
(one  half  toward  each  spindle  pole)  and  in  a  single  plane;  those  portions 
to  which  no  fibers  are  attached  may  extend  in  various  directions  with  do 
regular  arrangement. 

Anaphase  (Fig.  50,  B-D).- -The  daughter  chromosomes  (the  hah 
of  the  double  chromosomes  seen  at  metaphase)  now  begin  to  separate, 
first  at  the  point  of  insertion,  and  gradually  move  away  from  the  equa- 
torial plane.  Owing  to  the  different  locations  of  the  points  of  fiber  attach- 
ment, and  also  to  the  fact  that  the  free  ends  of  the  chromosomes  occupy 
various  positions,  the  chromosomes,  unless  they  are  wry  short,  may  new 

1  This  description  is  based  on  the  author's  accounts  of  somatic  mitosis  in  Vicia 
faba  (1913)  and  Tradescaniia  virginiana  (1920).  In  these  papers,  especially  in  the 
first,  there  is  presented  a  more  extensive  comparison  of  the  results  of  other  investi- 
gators than  can  he  given  here.  Comparative  studies  have  shewn  that  in  general 
the  present  description  is  widely  applicable  to  mitotic  phenomena  in  plants  and  ani- 
mals, although  many  modifications  in  detail  arc  known,  particularly  in  forms  with 
small  chromosomes.  A  useful  list  of  works  on  mitosis  in  angiosperms  is  given  by 
Picard  (1913). 


148 


INTRODUCTION  TO  CYTOLOGY 


be  drawn  into  a  number  of  peculiar  shapes.  In  the  case  of  long  chromo- 
somes the  portions  to  which  the  fibers  are  attached  may  have  reached 
the  poles  of  the  spindle  while  the  other  portions  are  not  yet  separated 
at  the  equatorial  plane.  As  soon  as  the  daughter  chromosomes  become 
entirely  free  from  one  another  they  quickly  draw  apart  and  contract 
into  two  dense  masses,  which  are  often  actually  farther  apart  than  were 


...  * 


i 


i 


- 

i 


I 

/  . 


V  *  » 

«    £     <a 


&&  a  •&& 


Fig.  50. — Somatic  mitosis  in  Tradescantia  virginiana:  metaphase  (A),  anaphase  (B-D), 
and  telophase  (E-G).  At  F  are  shown  cross  sections  of  chromosomes  in  the  stage  shown 
at  E.      X  1900.     {After  Sharp,  1920.) 


the  poles  of  the  spindle  at  metaphase.  In  these  masses  the  individual 
chromosomes  can  be  distinguished  only  with  great  difficulty  or  not  at  all. 
With  this  stage,  which  has  been  referred  to  by  Gregoire  and  Wygaerts 
(1903)  as  the  tassement  polaire,  the  anaphase  ends  and  the  telophase 

begins. 

Telophase  (Fig.  50,  E — Fig.  51,  7). — After  remaining  tightly  pressed 
together  for  a  short  time  the  chromosomes  of  each  daughter  group  begin 
to  separate,  their  individual  boundaries  again  becoming  visible.  As  they 
do  so  they  cohere  at  various  points  where  their  substance  becomes 
drawn  out  to  form  anastomoses.     It  seems  clear  that  the  main  connec- 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY       149 

tions  between  the  chromosomes  of  the  reorganizing  telophase  nucleus 
are  formed  in  this  way,  at  least  in  mitoses  showing  a  tassement  polaire 
stage;  but  it  is  also  probable,  as  several  investigators  have  pointed  out 
that  other  anastomoses  may  grow  out  from  one  chromosome  to  anothei 
in  the  manner  of  pseudopodia  (Boveri  1904;  Gates  1912;  Strasburger 
1905;  Dehorne  1911;  Muller  1912;  Lundegardh). 

Reactions  taking  place  between  the  various  chromosomes  and  espec- 
ially between  them  and  the  cytoplasm  now  result  in  the  productioD  of 
the  nuclear  sap,  or  karyolymph.  Between  the  outermost  chromosomes 
and  the  cytoplasm  and  also  within  the  chromosome  group  droplets  of 
clear  karyolymph  appear,  and  where  these  come  in  contact  with  the 
cytoplasm  a  nuclear  membrane  is  formed.  As  the  karyolymph  increases 
in  amount  the  nucleus  enlarg/es  and  the  chromosomes  become  more 
widely  separated. 

The  telophasic  alveolation  of  the  chromosomes,  although  it  may  in 
exceptional  cases  begin  much  earlier,  usually  commences  at  about  the 
time  the  chromosomes  first  separate  from  one  another  in   the  earlv 
reorganization  stages  of  the  daughter  nucleus.     Within  each  chromosome 
vacuoles  appear,  first  as  obscure  though  rather  sharply  delimited  circular 
or  elongated  areas.     They  lie  not  only  along  the  axis  but  also  near 
and  against  the  periphery.     This  point  is  of  importance  in  evaluating 
the  claim  advanced  by  certain  investigators   (Lundegardh  1910,   1912; 
Fraser  and  Snell  1911;  Fraser  1914;  Digby  1919)  that  the  vacuolation 
is  median  and  results  in  a  splitting  of  the  chromosomes  during  the  telo- 
phase, rather  than  in  the  prophase.     While  the  vacuoles  develop  into 
open  spaces  through  the  breaking  down  of  the  thin  portions  bounding 
them  the  nucleus  increases  rapidly  in  volume,  so  that  each  chromosome 
appears  as  an  irregular  net-like  band  joined  to  its  neighbors  by  fine  anas- 
tomoses.    Careful  study  of  the  details  of  these  telophasic  changes  (a 
cross  sections  of  chromosomes  in  Fig.  50,  F)  shows  that  the  alveolation 
proceeds  with  little  regularity,  and  that  each  chromosome  becomes  an 
alveolar  and  then  reticulate  body  with  nothing  which  can  properly  be 
called  a  longitudinal  split. 

In  certain  cases  these  internal  changes,  which  result  in  the  trans- 
formation of  the  chromosomes  into  a  reticulum,  and  which  as  a  rule  do  not 
begin  until  the  telophase,  may  be  initiated  during  the  anaphase.  In 
Allium,  for  instance,  Miss  Merriman  (1904),  Lundegardh  (1910,  19121 
and  Nemec  (1910)  all  report  that  the  vacuolation  of  the  chromosomes 
begins  at  this  time.  Even  more  striking  is  the  case  of  Trillium  (( iregoire 
and  Wygaerts  1903),  in  which  the  unusually  large  chromosomes  may 
show  vacuoles  as  early  as  the  metaphase  (Fig.  54,  -1).  Internal  changes 
of  other  types  have  also  been  described  in  anaphase  chromosomes,  but 
not  with  sufficient  clearness  to  warrant  their  use  in  general  interpre- 
tations. 


150 


ISTHODUCTION   TO  CYTOLOGY 


According  to  Bonnevie  (1908,  1911)  the  chromosomes  of  Allium, 
Ascaris,  and  Amphiuma  each  give  rise  to  an  endogenous  spiral  thread 
during  the  telophases,  this  spiral  thread  persisting  through  the  resting 
stages  until  the  next  prophase,  when  it  again  condenses  to  form  a  chromo- 
some (Fig.  54,  B).  In  his  work  on  Salamandra  Dehorne  (1911)  asserted 
that  each  chromosome  is  represented  at  telophase  by  two  interlaced 
spirals  arising  from  an  anaphasic  split,  and  further  that  these  double 
structures  are  associated  in  pairs  and  persist  in  this  condition  through 
the  resting  stages.  These  two  conceptions  have  been  criticized  by 
Gregoire  (1912),  Sharp  (1913),  and  de  Smet  (1914),  who  have  inter- 
preted such  appearances  as  occasional  aspects  of  the  alveolized  chromo- 
somes without  the  significance  attributed  to  them  by  Bonnevie  and 
Dehorne. 


H 


f^ 


& 


& 


■'7  ■ 


Fig.   51. — Somatic   mitosis  in    Tradescanlia  virginiana: 
phase  and  resting   stage    (/,    K),   and   early  prophase    (L, 
1920.) 


late   telophase    (H,   I),   inter- 
M).      X  1900.      {After  Sharp, 


In  the  young  telophasa  nucleus  the  chromosomes  may  become  ar- 
ranged in  the  form  of  a  more  or  less  continuous  daughter  spireme  which 
is  then  transformed  into  the  resting  reticulum.  This  spireme  stage, 
however,  is  not  a  necessary  one;  its  absence  is  being  reported  with 
sufficient  frequency  to  throw  much  doubt  upon  the  view  that  it  is  a 
phenomenon  of  even  general  occurrence. 

The  nucleolus  usually  makes  its  appearance  during  the  early  telophase 
as  a  small  droplet  or  as  several  such  droplets  which  may  later  flow 
together.  It  seems  to  have  little  direct  connection  with  the  chromo- 
somes, but  there  can  be  no  doubt  that  its  appearance  is  closely  associated 
with  their  physiological  activities. 

As  the  telophasic  changes  proceed  the  chromosomes  with  their 
anastomoses  gradually  form  a  more  and  more  uniform  reticulum,  in 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY       151 

which,    however,    the    limits   of    the    component  chromosomes    can    be 
distinguished  until  a  very  late  stage. 

Interphase  (Fig.  51,  J). -It  often  happens  thai  in  rapidly  growing 
tissue,  such  as  the  meristem  of  the  root  tip,  the  mitoses  succeed  one 
another  so  rapidly  that  the  telophasic  changes  may  nol  proceed  far 
enough  to  obscure  the  limits  of  the  chromosomes  in  the  reticulum 
before  the  changes  of  the  ensuing  prophase  begin.  In  Buch  tissue  n  is 
not  always  possible  to  tell  whether  a  given  nucleus  will  undergo  further 
telophasic  change  or  will  at  once  enter  upon  the  prophases.  Such 
interphasic  nuclei  develop  nucleoli,  but  karyosomes  (in  species  which 
have  these  bodies)  are  usually  not  formed  until  a  more  advanced  stage. 
Resting  Stage  (Fig.  51,  J,  X).— In  slowly  growing  tissue  the  successive 
mitoses  do  not  follow  one  another  with  very  great  rapidity,  and  the 
telophasic  changes  are  carried  on  until  the  condition  characteristic  of 
the  typical  resting  nucleus  is  reached:  the  interphase  here  becomes  the 
prolonged  resting  stage.  The  structure  of  the  resting  nucleus  has  been 
fully  described  in  Chapter  IV.  In  the  reticulum  the  limits  of  the  con- 
stituent chromosomes  usually  become  indistinguishable,  although  n  i- 
known  that  in  certain  cases  such  nuclei,  if  properly  sectioned  and  stained, 
may  reveal  heavier  and  lighter  areas  in  the  reticulum  which  represent 
respectively  the  chromosomes  and  the  regions  of  anastomosis  between 
them.  The  importance  of  these  facts  will  be  apparent  in  our  treatment 
of  the  individuality  of  the  chromosomes. 

Prophase  (Fig.  51,  L-Fig.  52).— The  first  indication  that  the  prophasic 
changes  have  begun  is  seen  in  the  breaking  down  of  the  reticulum  in 
certain  regions.  In  the  case  of  nuclei  which  show  heavier  and  lighter 
areas  in  their  reticula  this  breaking  down  occurs  along  the  light  portions. 
In  view  of  what  has  been  said  concerning  the  origin  of  the  reticulum  at 
telophase  it  is  apparent  that  the  breaking  up  of  the  reticulum  in  the 
prophase  represents  in  such  cases  the  separation  of  the  constituent 
chromosomes  from  each  other  along  the  lines  of  their  telophasic  union. 
and  it  has  been  inferred  that  a  similar  interpretation  applies  to  tic 
nuclei  in  which  the  reticulum  is  perfectly  uniform  or  in  which  tic  nuclear 
material  assumes  more  irregular  forms.  In  this  way  there  are  developed 
from  the  resting  reticulum  a  number  of  more  or  less  distinct  reticulate 
units,  which,  in  view  of  their  subsequent  behavior,  we  know  to  be  the 
chromosomes  (Fig.  51,  L,  M).  Thai  these  units  are  essentially  the  same 
as  those  which  went  to  make  up  the  ret  iculum  at  t  he  preceding  telophase 
seems  highly  probable;  there  can  be  little  doubt  on  tin-  poinl  when  the 
interphase  is  short. 

The  material  of  each  reticulate  unit  (chromosome    now  gradually  con- 
denses in  a  very  irregular  fashion   about    its  open   spaces  and   civile 
The  thinner  regions  bounding  these  spaces  and  cavities  become  broken 
down,  and  the  thicker  portions  remain  as  a  very  irregular  zigzag  thread  of 


152  INTRODUCTION  TO  CYTOLOGY 

uneven  thickness,  which  soon  begins  to  straighten  out  (Fig.  52,  P).  At 
the  same  time  the  material  composing  the  thread  becomes  more  evenly 
arranged  throughout  its  length,  so  that  the  chromosome  eventually  takes 
the  form  of  a  single  slender  thread.  All  of  these  changes — condensa- 
tion, straightening,  and  equalization  in  thickness — may  be  seen  going 
on  simultaneously  in  different  chromosomes  of  the  same  nucleus,  or  even 
in  different  portions  of  a  single  chromosome. 

The  formation  of  the  slender  prophase  chromosomes  from  the  retic- 
ulum in  the  above  manner  was  first  described  in  detail  by  Gregoire  and 
Wygaerts  (1903)  and  Gregoire  (1906),  and  new  cases  have  since  been 
added.     The  above  writers,  together  with  Nemec  (1910),  Digby  (1910), 


A      rv 


J 


N 


Ji 


R     *<<? 


Fig.  52.- — Somatic  mitosis  in  Tradescantia  virginiana:  prophases. 

At  N  are  shown  cross  sections  of   chromosomes  in   the  stage   shown  in   Fig.   51,   M. 
X  1900.     {After  Sharp,  .1920.) 

and  Miiller  (1912),  believe  that  the  separated  portions  of  the  reticulum 
may  also  condense  directly  into  the  slender  threads  without  passing 
through  the  very  irregular  zigzag  stage  above  described.  It  is  probable 
that  both  methods  are  followed  in  different  cases,  direct  condensation 
possibly  being  the  rule  in  small  nuclei.  The  view  of  Bonnevie  (1908, 
1911)  concerning  the  origin  of  the  zigzag  threads  has  already  been 
mentioned  in  the  paragraphs  on  the  telophase.  According  to  this  worker 
and  to  certain  others  (Wilson  1912a6)  the  chromatic  material  forms  a 
spiral  thread  within  the  chromosome  during  the  telophase,  this  thread 
uncoiling  and  emerging  from  the  chromosome  in  the  following  prophase. 
It  is  true  that  the  zigzag  threads  occasionally  have  a  strikingly  regular 
spiral  aspect,  but  in  view  of  the  many  other  aspects  observed  and  the 
process  which  is  known  to  give  rise  to  them,  it  is  probable  that  the 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY  In- 
formation of  spirals  in  the  manner  described  by  Bonnevie  is  at  l,-,  , 
very  exceptional. 

The  true  longitudinal  splitting  of  the  chromosomes  is  initiated  in 
the  slender  threads  of  the  early  prophase  (Figs.  52  and  53,  Q)  \.  800n 
as  a  thread  becomes  sufficiently  equalized  in  diameter  small  vacuoles 
appear  along  its  axis  and  rapidly  develop  into  a  more  or  less  continues 
split.  Not  all  the  threads,  nor  even  all  portions  of  ,1,,  same  thread 
undergo  the  change  at  the  same  time.  If  we  consider  tin-  whole  nucleus 
at  once,  the  processes  of  condensation,  straightening,  equalization    and 


Q 


r**^ 


u 

Fig.  53. — Somatic  mitosis  in    Vivid  fab  a:  prophases. 
^^Stages  P,  Q,  and  S  correspond  with  P,  Q,  and  S  of   Fig.  52.      X  L650.      (After  Sharp. 

splitting  are  all  going  on  simultaneously;  only  in  a  given  small  portion  of 
a  thread  do  they  follow  in  definite  sequence.  Furthermore,  as  soon  as 
the  threads  become  equalized  they  at  once  begin  to  shorten  and  thicken, 
so  that  when  vacuolation  and  splitting  are  a  little  delayed  they  occur  in 
somewhat  heavier  threads. 

The  manner  in  which  the  vacuoles  develop  into  the  complete  split 
should  be  carefully  noted.  The  vacuoles  quickly  form  openings  which 
extend  completely  through  the  chromosome,  so  thai  the  latter  soon  takes 
the  form  of  two  parallel  strands  connected  l>y  heavy  cross  pieces  repre- 
senting the  portions  between  the  original  vacuoles    I'm-.  .".J  and  53,  S 


154  INTRODUCTION  TO  CYTOLOGY 

The  materia]  constituting  the  cross  pieces  gradually  moves  to  the  two 
side  strands,  the  center  portion  of  the  cross  piece  becoming  progressively 
thinner  and  the  material  accumulating  on  the  side  strands  as  a  pair  of 
chromatic  lumps.  Although  some  of  the  cross  pieces  may  persist  until  a 
relatively  late  stage  most  of  them  soon  disappear  completely,  and  the 
material  in  the  two  chromatic  lumps  is  gradually  distributed  more  or 
less  evenly  along  the  parallel  strands,  which  represent  the  daughter 
chromosomes  resulting  from  the  split. 

The  double  chromosomes  now  shorten  and  thicken,  forming  the 
'thick  spireme"  so  conspicuous  in  prophase  nuclei  (Fig.  53,  T,  U).  As 
pointed  out  in  the  preliminary  sketch  of  mitosis,  the  chromosomes  in  the 
prophase  may  form  a  more  or  less  continuous  spireme,  but  it  is  becoming 
increasingly  apparent  that  this  is  not  a  universal  phenomenon.  It  is 
certain  that  in  many  cases  the  chromosomes  are  separate  from  the  first, 
and  it  seems  therefore  that  any  association  in  the  form  of  a  continuous 
spireme  is  a  matter  of  secondary  importance.  As  the  shortening  and 
thickening  proceed  the  split  may  become  obscured  by  the  close 
association   of   the   halves,   but   suitable   methods   reveal   its   presence. 

While  indications  of  spindle  formation  are  appearing  in  the  cytoplasm 
the  nucleolus  disappears  and  the  nucleus  begins  to  contract,  so  that  the 
thick  double  chromosomes  become  very  closely  packed  together.  While 
the  contraction  is  at  its  height  the  nuclear  membrane  disappears,  after 
which  the  chromosomes  loosen  up  as  an  irregularly  arranged  group. 
This  contraction  stage  evidently  does  not  occur  in  many  mitoses:  the 
membrane  may  disappear  while  the  nucleus  has  its  full  size.  However, 
when  it  does  occur  it  is  of  very  short  duration,  so  that  it  may  take  place 
in  more  cases  than  has  been  supposed.  After  the  disappearance  of  the 
nuclear  membrane  the  spindle  fibers  establish  connection  with  the  chro- 
mosomes, which  quickly  become  arranged  with  their  halves  in  superposi- 
tion at  the  equatorial  plane,  as  described  in  the  paragraph  on  the 
metaphase.  This  brings  us  to  the  point  with  which  our  description 
began. 

It  should  be  added  that  in  many  descriptions  of  mitosis,  notably 
those  presented  in  general  text  books,  the  chromosomes  are  said  to  split 
during  the  metaphase,  after  they  have  become  arranged  upon  the  spindle. 
Such  a  late  development  of  the  split  may  indeed  occur  in  some  cases,  but 
it  is  not  improbable  that  closer  examination  would  often  reveal  the 
inception  of  the  process  at  a  much  earlier  stage.  As  has  been  pointed 
out  in  the  foregoing  description,  the  early  formed  split  frequently  be- 
comes obscured  during  the  later  prophases  owing  to  the  shortening  and 
thickening  of  the  chromatin  threads,  and  becomes  conspicuous  again 
only  after  the  metaphase  figure  has  been  established. 

Chromomeres. — One  matter  which  should  receive  special  attention  is 
that  of  the  chromomeres.     It  was  held  by  Roux  (1883)  that  the  compli- 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY 


1  ."»."> 


cated  process  of  mitosis  is  meaningless  unless  iln'  chromatin  is  quali- 
tatively  different  in  the  various  regions  of  the  nucleus,  and  thai  the 
arrangement  of  the  material  of  the  chromosome  in  the  form  of  a  long 

thread  prior  to  its  splitting;  is  a  means  whereby  all  these  qualities,  ar- 
ranged in  a  linear  series  in  the  thread,  are  equationally  divided  and 
distributed  to  the  daughter  nuclei.  The  theory  of  Balbiani  I  L876)  and 
Pfitzner  (1881),  that  the  chromatin  granules  visible  in  the  nuclear  rel  icu- 
lum  arrange  themselves  in  a  series  in  the  chromosome  and  by  their 
division  initiate  its  splitting,  had  much  to  do  with  the  formulal  ion  of  thia 
hypothesis.  That  the  chromatic  granules,  or  chromomeres  (Fol  L891), 
represent  the  qualities  of  Roux  is  a  theory  which  has  been  widely  accepted 
by  cytologists.  It  was  the  opinion  of  Braucr  (1893)  and  many  later 
workers  that  the  granules  or  chromomeres,  rather  than  the  chromosomes 
themselves,  are  the  significant  units  in  the  nucleus,  and  that  their  division 
is  an  act  of  reproduction.  The  division  and  separation  of  chromosomes 
was  accordingly  regarded  as  a  means  of  distributing  the  daughter  granules 
to  the  daughter  cells.  That  the  chromomere  is  made  up  of  still  smaller 
"chromioles"  was  held  by  Eisen  (1899,  1900).  Strasburger,  Allen  (1905 
and  Mottier  (1907)  also  found  the  chromomere  to  be  composed  of  smaller 
chromatic  granules. 


r\A  fee? 


Fig.   54. 
A,  vacuoles  in  chromosomes  at  metaphase  in  Trillium.      X  1800.      (After  Gregoin  and 
Wygaerts,  1903.)     B,  spiral  arrangement  of  chromatin  material  within  the  chromosomes 
Allium.     (After  Bonnevie,  1911.)     C,  D,  stages  of  chromosome  splitting  in  Naja*  marina, 

showing  chromomeres.      X  2250.      (After  MiOler,  1912.) 


Although  a  large  number  of  investigators,  particularly  those  interested 
in  the  hereditary  role  of  the  chromatin,  have  placed  much  confidence  in 
the  importance  of  the  chromomeres  (Strasburger  L884,  lsss  ,  others 
have  raised  serious  objections  to  the  theory  that  they  arc  significant  units 
or  individuals.  Gregoire  and  Wygaerts  (1903),  .Martins  Mano  (1904 
Gregoire  (1906,  1907),  Marechal  (1907).  Bonnevie  (1908),  Stomps 
(1910),  Lundegardh  (1912),  Sharp  (1913,  L920),  and  others  have  found 
no  such  definite  behavior  on  the  part  of  the  chromatin  granules  in  the 
dividing  chromosomes  studied  by  them,  and  have  suggested  other  ex- 
planations for  the  appearances  observed.  According  to  a  modification 
of  the  chromomere  theory  adopted  by  Muller  I  L912)  the  portions  of  the 
thread  between  the  chromomeres  split  first,  the  division  of  thechromo- 


156  INTRODUCTION  TO  CYTOLOGY 

meres  then  following.  It  has  been  pointed  out  (Sharp  1913)  that  Muller's 
figures  (Fig.  54,  C,  D),  which  are  very  similar  to  the  later  ones  of  Stras- 
burger  (1907),  may  be  interpreted  as  steps  in  the  division  of  a  homogene- 
ous chromatic  thread  by  the  formation  of  vacuoles,  and  that  the 
chromomeres  in  this  case  are  merely  the  cross  pieces  between  the  halves 
of  the  incompletely  split  chromosome,  as  described  in  the  foregoing  account 
of  the  prophase  (Fig.  53,  S). 

It  is  becoming  increasingly  apparent  that  the  distinction  between 
chromatin  granules  and  supporting  thread  is  not  so  sharp  as  has  been 
supposed,  since  the  chromatic  substance  is  often  very  fluid  in  consist- 
ency; and  many  have  felt  that  the  granules  when  present  are  far  too 
inconstant  in  number  and  behavior  to  serve  as  the  ultimate  units  which 
students  of  heredity  hope  to  find.  On  the  other  hand,  it  should  be  said 
that  the  constancy  in  size  and  position  of  the  chromomeres  described 
by  Wenrich  (1916)  for  the  grasshopper,  Phrynotettix  (Fig.  155),  argues 
strongly  for  the  hereditary  significance  of  these  bodies,  some  of  which 
can  be  seen  to  retain  their  identity  through  the  resting  stages.  But 
whatever  their  importance  may  be,  the  arrangement  of  the  chromatic 
material  in  the  form  of  a  long  slender  thread  and  its  accurate  splitting 
into  exactly  similar  halves  are  very  suggestive  in  connection  with  the 
theory  of  Roux  that  many  qualities  are  arranged  in  a  row  and  all 
divided  at  the  time  of  nuclear  and  cell  division.  This  subject  will 
receive  further  attention  in  the  chapters  dealing  with  heredity. 

Summary. — The  chromosomes,  after  having  arrived  at  the  poles  of  the 
achromatic  figure,  become  irregularly  alveolized  during  the  telophase  and 
form  ragged  net-like  structures.  These  are  joined  to  each  other  by  fine 
anastomoses  and  so  make  up  the  continuous  reticulum  of  the  resting 
stage.  In  the  next  prophase  this  reticulum  breaks  up  into  separate 
small  nets  or  alveolar  units,  each  of  which  represents  a  chromosome. 
The  units  condense  in  a  peculiar  manner  and  become  long  slender  threads. 
These  threads  undergo  a  longitudinal  splitting.  The  double  threads  so 
formed  shorten  and  thicken,  and  become  the  double  chromosomes  which 
are  arranged  on  the  spindle  at  metaphase.  The  two  halves  (daughter 
chromosomes)  making  up  each  double  chromosome  separate  and  pass  to 
opposite  poles  during  the  anaphase. 

The  outstanding  and  significant  feature  of  somatic  mitosis  is  this: 
each  chromosome  is  accurately  divided  into  two  exactly  equal  longitudinal 
halves  which  are  distributed  to  the  two  daughter  nuclei.  The  two  daughter 
cells  thus  receive  exactly  similar  halves  of  the  chromatin  of  the  mother  cell. 
Furthermore,  as  will  be  shown  below,  there  is  good  evidence  for  the  view 
that  the  chromosomes  maintain  an  individuality  of  some  sort,  so  that, 
since  all  the  nuclei  of  the  body  arise  by  the  repeated  equational  division 
of  a  single  nucleus,  all  the  somatic  (body)  cells  are  qualitatively  similar  in 
chromatin  content:  they  contain  representatives  or  descendants  of  each  and 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY        157 

every  chromosome  present  in  the  first  cell  of  the  series.  The  greal  i  heorel  ical 
importance  of  these  facts  will  be  apparent  when  we  take  up  the  subject 
of  chromosome  reduction,  and  the  application  of  cytological  phenomena 

to  the  problems  of  heredity. 

THE  INDIVIDUALITY  OF  THE  CHROMOSOME 

In  later  chapters  the  question  of  the  significance  of  the  ouclear  struc- 
tures in  heredity  is  to  be  considered.  In  connection  with  this  question 
it  is  of  the  highest  importance  to  determine  whether  or  not  the  chromo- 
somes to  which  the  reticulum  gives  rise  in  the  prophase  are  in  any  real 
sense  the  same  as  those  which  went  to  make  up  the  reticulum  at  the  pre- 
ceding telophase.  That  they  do  preserve  their  identity  as  individuals 
through  the  resting  stage,  arise  only  by  division,  and  maintain  therefore 
a  genetic  continuity  throughout  the  life  cycle,  was  held  by  van  Beneden 
(1883),  Rabl  (1885)  and  Boveri  (1887,  1888,  1891)  many  years  ago,  and 
since  that  time  the  idea  has  received  the  support  of  a  large  number  of 
investigators.  We  shall  now  briefly  review  some  of  the  evidences  which 
have  led  the  majority  of  cytologists  to  the  view  that  the  chromosomes, 
"if  .  .  .  not  actually  persistent  individuals,  as  Rabl  and  Boveri  have 
maintained,  .  .  .  must  at  least  be  regarded  as  genetic  homologies  that 
are  connected  by  some  definite  bond  of  individual  continuity  from  gen- 
eration to  generation  of  cells"  (Wilson  1909). 

The  Frequent  Persistence  of  Visible  Chromosome  Limits  in  the 
Resting  Reticulum. — In  the  foregoing  description  of  the  behavior  of  the 
chromosomes  in  mitosis  it  was  pointed  out  that  in  rapidly  dividing  I  issue 
the  telophasic  alveolation  of  the  chromosomes  and  their  anastomosis  to 
form  the  reticulum  often  do  not  proceed  far  enough  during  the  interphase 
to  obliterate  the  boundaries  between  the  chromosomes,  which  separate 
again  in  the  ensuing  prophase  without  having  lost  their  visible  identity. 
In  such  nuclei  there  can  be  little  doubt  that  the  autonomy  of  the  chromo- 
some is  preserved.  In  other  cases,  however,  the  telophasic  transforma- 
tion of  the  chromosomes  is  more  complete  and  the  resulting  reticulum 
reacts  very  weakly  to  the  stains,  so  that  the  limits  of  the  constituent 
chromosomes  disappear  from  view  completely.  Many  workers  have 
therefore  objected  to  the  statement  that  here  also  the  chromosomes  are 
present  as  individuals,  although  invisible.  Haecker  (1902)  and  Boveri 
(1904)  pointed  out  that  this  objection  may  be  met  by  assuming  that  it  is 
the  achromatic  framework  of  the  alveolized  chromosome,  and  not  nee 
sarily  the  basichromatic  fluid  held  within  it,  that  maintains  a  structural 
independence.  This  view  had  the  support  of  the  earlier  observation 
made  by  Boveri  (1887a,  1888a,  1891;  also  1909)  and  confirmed  by  Herla 
(1893),  that  the  chromosomes  in  the  segmenting  egg  of  Ascaris  have  a 
certain  arrangement  when  they  build  up  the  nuclear  reticulum  in  the 
telophase  and  reappear  from  the  reticulum  in  the  same  position  at  the 
next  prophase. 


1 58 


INTRODUCTION  -TO  CYTOLOGY 


Special  emphasis  was  laid  upon  this  interpretat  ion  by  Marechal  (1904, 
1907)  as  a  result  of  his  studies  on  the  growth  stage  of  animal  oocytes. 
At  this  period  in  the  development  of  the  ovum  the  chromosomes  assume 
a  finely  branched  form  (Fig.  86,  C,  D)  and  their  ordinary  staining  capacity 
is  lost  completely.  Although  the  chromatic  fluid  may  flow  from  the 
reticulum  to  the  nucleolus  and  vice  versa,  and  may  periodically  undergo 
chemical  changes  which  radically  alter  its  staining  reactions,  the  achro- 
matic chromosomal  substratum  nevertheless  maintairis  an  uninterrupted 
structural  continuity.  Such  a  transfer  of  the  basichromatic  material 
from  the  persistent  reticulum  to  the  nucleolus  during  the  telophase,  and 
to  the  reticulum  again  during  the  succeeding  prophase,  has  also  been 


Fig.    55. — Some    evidences   for    chromosome    individuality. 
A,    chromosomal   vesicles   in   Brachystola   magna;   x-chromosome   in   vesicle   at   right. 
(After  Sutton,  1902.)      B,  chromosomal  vesicles  in  Fundulus  embryo.      X  c.  1800.      (After 
Richards,   1917.)      C,   chromomere  vesicle   (c)    on  chromosome  of  Chorthippus.      X  1500. 
(After  Wenrich,  1917.)      D,  prochromosomes  in  Pinguicula.      X  4200. 

observed  by  Strasburger  (1907)  and  Berghs  (1909)  in  the  somatic  nuclei 
of  Marsilia  (Fig.  17,  E).  The  chromosome,  as  Marechal  urges,  is  not 
simply  a  mass  of  chromatin,  but  rather  "a  structure  periodically  chro- 
matic;" hence  the  disappearance  of  stainable  substance  does  not  signify 
the  loss  of  structural  continuity  on  the  part  of  the  chromosome. 

The  "chromosomal  vesicles"  (Fig.  55,  A,  B)  observed  by  certain 
investigators  constitute  valuable  evidence  in  this  connection.  In  the 
spermatogonia  of  the  grasshopper,  Phrynotettix,  for  example,  Wenrich 
1916)  has  shown  that  each  of  the  alveolizing  chromosomes  forms  its 
own  vesicle  about  it  at  telophase,  the  several  vesicles  joining  to  form  a 
common  nucleus.  In  some  cases  the  boundaries  between  the  vesicles  do 
not  entirely  disappear  during  the  resting  stages,  and  at  the  next  prophase 


SOMATIC  MITOSIS  AND  CHROMOSOME  I.\D/\  TDUALITY        I.V.I 

the  chromatic  material  of  each  vesicle  organizes  in  the  form  of  a  chromo- 
some. The  same  condition  is  found  in  the  nuclei  of  Fundulus  (Richards 
1917),  Crepidula  (Conklin  1902),  and  certain  fish  hybrids  (Pinney  }<)\^ 
From  this  it  is  evident  that  the  morphological  identity  <>f  the  chromo- 
somes  has  not  been  lost  between  mitoses,  although  ;i  very  different  type 
of  organization  has  been  assumed. 

In  Carex  aquatilis  Stout  (1912)  has  found  a  peculiar  condition,      line 
the  very  small  spherical  chromosomes,  which  maintain  a  serial  arranj 
ment,  are  visible  in  the  resting  state,  and  can  be  traced  continuously 
through  all  stages  of  the  somatic  and  germ  cell  divisions  witli  the  excep- 
tion of  synizesis. 

The  interpretations  of  Bonnevie  (1908,  1911)  and  Dehorne  (1911  . 
according  to  whom  the  chromosomes  persist  through  the  resting  stage  as 
spirals  or  double  spirals,  have  been  mentioned  in  the  description  <»l 
mitosis. 

Prochromosomes. — Bodies  known  as  prochromosomes  have  been 
described  in  the  nuclei  of  a  number  of  plants:  in  Thalictrum,  Calycanthus, 
Campanula,  Helleborus,  Podophyllum,  and  Richardia  by  Overton  (1905, 
1909) ;  in  the  Cruciferae  by  Laibach  (1907) ;  in  Drosera  and  other  forms  by 
Rosenberg  (1909);  in  Acer  platanoides  by  Darling  (1914);  in  Musa  by 
Tischler  (1910);  and  in  a  number  of  other  forms.  These  prochromosomes 
appear  as  small  chromatic  masses  in  the  reticulum  (Fig.  55,  //),  and 
correspond  approximately  in  number  to  the  chromosomes  of  the  species. 
They  are  generally  looked  upon  as  portions  of  chromosomes  which  have 
not  undergone  complete  alveolation,  and  as  centers  about  which  the 
chromosomes  again  condense  at  the  next  prophase.  This  interpret  at  inn  is 
in  all  probability  a  valid  one  in  many  of  the  described  cases,  but  in  <>t  hers 
the  significance  of  such  chromatic  masses  is  questionable.  In  Cn  , 
virens  de  Smet  (1914),  in  harmony  with  the  conclusions  of  Miss  Digby 
(1914),  finds  them  to  be  accumulations  of  material  formed  during  tic 
resting  stages.     If  such  is  the  case  they  are  to  be  regarded  as  karyosom< 

Persistence  of  Parental  Chromosome  Groups  After  Fertilization.  In 
Chapter  XII  it  will  be  shown  that  at  fertilization  there  are  brought 
together  two  sets  of  chromosomes,  one  set  from  each  parent ;  and  that  in 
every  nucleus  of  the  resulting  individual  the  chromosomes  furnished  by 
the  two  parents  are  present  together,  all  of  them  dividing  at  every  mitosis. 
When  the  chromosomes  of  the  male  parent  are  similar  to  those  of  the 
female  parent  it  is  usually  impossible  to  distinguish  them  in  the  nuclei 
of  the  offspring.  In  a  number  of  cases,  however,  such  as  Crepidula 
(Conklin  1897,  1901),  Cyclops  (Haecker  L895;  Ruckeri  L895  .  and  Crypto- 
branchus  (Smith  1919)  (Fig.  109),  the  two  parental  groups  are  distinguish- 
able on  the  mitotic  spindle,  and  often  at  other  stages,  through  several 
embryonal  cell  generations.  It  is  in  hybrids  that  this  phenomenon  i- 
shown  most  strikingly.     In  hybrid  fishes  obtained  by  crossing  Fundulus 


160  INTRODUCTION  TO  CYTOLOGY 

with  Menidia  Moenkhaus  (1904)  was  able  to  distinguish  easily  between 
the  long  (2.18  ix)  chromosomes  of  Fundulus  and  the  short  (1m)  ones  of 
Menidia.  Here,  as  in  Crepidula  and  Cyclops,  the  paternal  and  maternal 
chromosomes  form  separate  groups  in  the  mitotic  figure.  A  similar 
condition  was  seen  by  Tennent  (1912)  in  hybrid  echinoderms  obtained  by 
crossing  in  various  ways  Moira,  Toxopnenstes,  and  Arbacia.  In  the 
later  cell-divisions  the  parental  chromosomes  mingle  more  or  less,  but  are 
nevertheless  distinguishable.  In  Fundulus  X  Ctenolabrus  hybrids  (Morris 
1914;  Richards  1916),  as  well  as  in  the  normally  fertilized  Cryptobranchus 
(Smith  1919),  the  chromatin  contributions  of  the  two  parents  are  dis- 
tinguishable even  in  the  resting  nuclei. 

Size  and  Shape  of  Chromosomes.- — One  of  the  most  striking  evidences 
favoring  the  theory  of  individuality  has  been  found  in  those  plants  and 
animals  which  show  constant  differences  in  size  and  shape  among  the 
various  members  of  each  parental  chromosome  group,  so  that  particular 
chromosomes  are  recognizable  in  the  group  appearing  at  each  mitosis. 
Since  each  parent  furnishes  a  set  of  chromosomes  to  the  new  individual, 
each  kind  of  chromosome  is  present  in  duplicate  in  the  nuclei  of  this  indi- 
vidual: it  is  therefore  customary  to  speak  of  them  as  being  present  in 
pairs,  although  at  most  stages  of  the  life  history  there  is  ordinarily  no 
actual  spatial  pairing. 

Since  the  description  of  the  chromosomes  of  Brachystola  by  Sutton  in 
1902  (Fig.  101)  the  reported  cases  in  which  the  different  pairs  of  the 
chromosome  complement  possess  different  characteristic  sizes  and  shapes 
have  become  increasingly  numerous.  This  is  notably  true  of  insect 
cytology,  as  is  evident  in  a  review  of  the  extensive  researches  of  McClung 
(1905,  1914,  1917),  Robertson  (1916),  Harman  (1915),  Carothers  (1917), 
and  many  others.  In  the  sea  urchin,  Echinus,  Baltzer  (1909)  found  that 
the  36  chromosomes  have  constant  differences  in  length  and  shape, 
some  being  hooked  and  some  horseshoe-shaped.  In  the  flatworm, 
Gyrodactylus,  (Gille  1914)  there  are  six  pairs,  all  different  in  length.  In 
Ambystoma  tigrinum  Parmenter  (1919)  finds  14  pairs  of  graded  sizes. 
In  plants  may  be  cited  the  cases  of  Crepis  virens  (Rosenberg  1909;  de 
Smet  1914;  M.  Nawaschin  1915)  (Fig.  56  bis,  A),  which  has  three  pairs  of 
different  size;  Vicia  faba  (Sharp  1914;  Sakamura  1915),  with  five  short 
pairs  and  one  long  pair  (Fig.  56);  and  Najas  (Tschernoyarow  1914),  in 
which  there  are  seven  distinguishable  pairs  (Fig.  56  bis,  B) .  In  Najas 
the  smallest  pair  is  attached  to  one  of  the  larger  pairs:  Sakamura  (1920) 
thinks  that  these  together  are  really  a  single  pair  with  pronounced 
constrictions. 

Not  only  may  certain  chromosomes  be  distinguished  on  the  basis  of 
comparative  length,  but  in  some  cases  there  may  be  other  characteristics 
which  serve  as  marks  of  identification.  In  the  chromosomes  of  many 
plants  and  animals  there  are  pronounced  constrictions  in  some  of  the 


SOMATIC  MITOSIS  AXD  CHROMOSOME  INDIVIDUALITY         161 


f»f 


B 


a 


Fig.  56. — The  chromosome  complement  of  Vicia  faba. 

A,  B,  two  successive  sections  of  a  mitotic  figure  in  the  root  tip,  showing  together  the 
12  split  chromosomes,  2  of  them  about  twice  as  long  as  the  other  10.  C,  cross  section  of 
the  group  of  chromosomes  at  anaphase:  each  of  the  long  chromosomes,  beiim  drawn  pole- 
wards by  the  middle,  shows  both  ends,  making  the  number  apparently  14.  I).  K.  two 
successive  sections  of  a  heterotypic  figure  in  the  mierosporocyte,  showing  the  6  bivalents; 
the  large  one  is  at  the  left.  F,  polar  view  of  heterotypic  mitosis  at  metaphase,  showing  the 
6  bivalents.      X  1400.      (Original.) 


B 

Fig.   56  bis. 

A,  anaphase  of  somatic  mitosis  in  Crepis  virene,  showing  2  long,  2  medium  Bised,  and  2 
short  chromosomes  passing  to  each  pole.      (After  Rosenberg,   L920.)      B,  the  chromosome 

complement  in  a  somatic  cell  of  Najas  major,  showing  the  7  homologous  pairs.      {After 

Tschcrnoyarow,  1914.) 


11 


162  INTRODUCTION  TO  CYTOLOGY 

members  of  the  group.  It  has  been  shown  in  certain  instances  that  these 
constrictions  have  constant  positions  in  the  chromosome.  A  careful 
study  of  this  phenomenon  has  been  made  by  Sakamura  (1915,  1920). 
In  Viciafaba,  for  example,  he  finds  that  each  of  the  two  long  chromosomes 
("M-chromosomes")  of  the  somatic  group  has  two  constant  constrictions, 
one  at  the  middle  and  one  near  the  end  ("m-constriction"  and  ue-con- 
striction")  (Fig.  56,  A).  The  m-constriction  marks  the  point  of  attach- 
ment of  the  spindle  fibers.  There  are  also  end-constrictions  in  8  of  the  10 
short  chromosomes.  On  the  basis  of  the  widespread  occurrence  of  con- 
strictions in  the  chromosomes  of  both  plants  and  animals  Sakamura  has 
interpreted  a  number  of  puzzling  phenomena,  such  as  the  apparent  vari- 
ation in  chromosome  number  within  the  species  (see  below)  and  certain 
features  of  the  reduction  process  (Chapter  XI) 

Such  regularly  situated  constrictions  have  also  been  demonstrated  in 
Fritillaria  tenella  by  S.  Nawaschin  (1914).  Here  they  are  present  at  the 
middle  of  the  largest  chromosomes,  nearer  one  end  in  the  medium-sized 
chromosomes,  and  close  to  the  end  of  the  smallest  ones.  In  Crepis  virens 
(M.  Nawaschin  1915)  there  are  constrictions  near  one  end  in  two  of  the 
1  hree  chromosomes  of  the  haploid  group  in  the  pollen  grain,  in  four  of  the 
six  chromosomes  of  the  diploid  group  in  the  somatic  cells,  and  in  six  of 
the  nine  chromosomes  of  the  triploid  group  in  the  endosperm  cells.  Such 
a  definiteness  in  the  location  of  constrictions  was  also  seen  earlier  by  Agar 
(1912)  in  the  chromosomes  of  the  fish,  Lepidosiren. 

Somewhat  similar  evidence  has  been  brought  forward  by  Wenrich 
(1916),  who  finds  that  the  chromatic  lumps,  or  chromomeres,  have  a 
striking  constancy  in  position  as  well  as  in  size  in  the  chromosomes  of 
Phrynotettiz  (Fig.  155).  Wenrich  (1917)  also  reports  that  the  small 
"chromomere  vesicles"  attached  to  the  chromosomes  of  certain  orthop- 
terans  always  appear  at  definite  points  along  the  chromosome  (Fig.  55,  C). 
It  therefore  appears  that  the  chromosomes  of  a  given  group  or  comple- 
ment not  only  maintain  a  genetic  continuity  from  cell  to  cell,  but  are  also 
in  some  way  qualitatively  different  from  one  another.  They  are  conse- 
quently said  to  have  a  specificity  as  well  as  an  individuality,  or  continuity. 
The  relatively  constant  positions  of  the  constrictions,  chromatic  lumps, 
and  chromomere  vesicles  afford  further  visible  evidences  that  the  chrom- 
osome may  possess  some  kind  of  lengthwise  differentiation,  a  fact  which, 
if  clearly  demonstrated,  would  be  of  the  highest  importance  in  connection 
with  current  views  of  the  role  of  the  chromosomes  in  heredity.  (See 
Chapter  XVII.)  The  significance  of  chromosome  constrictions  in  this 
respect  has  been  emphasized  by  Janssens  (1909),  S.  Nawaschin  (1915), 
and  Sakamura  (1920). 

Chromosome  Number.1 — It  was  long  ago   noticed  by  Boveri,  van 

1  For  lists  of  chromosome  numbers  in  plants  see  Ishikawa  (1916)  and  Tisehler 
(1916).     For  the  numbers  in  animals  see  Harvey  (1916,  1920). 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY       163 

Bcneden,  and  Strasburger  that  the  number  of  chromosomes  in  any  given 
species  is  relatively  constant.  It  was  largely  upon  this  fad  thai  the 
theory  of  chromosome  individuality  was  originally  based:  the  fad  thai 
the  number  of  chromosomes  appearing  at  every  mitosis  is  almosl  invari- 
ably the  same  was  taken  to  mean  that  the  structural  identity  of  the 
chromosomes  is  never  lost.  Certain  observers  (Fick  1905,  1909)  have 
held  that  the  apparent  constancy  in  number  is  not  due  to  a  structural 
continuity  or  individuality  of  any  sort,  but  rather  to  the  fact  thai  the 
successive  nuclei  have  a  relatively  uniform  amount  of  nuclear  material, 
the  chromosomes  "crystallizing  out"  of  this  material  in  each  prophase 
and  going  into  solution  at  the  close  of  mitosis.  This  idea  was  especially 
developed  by  Delia  Valle  (1909,  1912a&),  who  described  the  formation 
of  chromosomes  by  the  aggregation  of  fluid  crystals  during  the  prophase. 
These  chromosomes  he  held  to  be  in  no  sense  morphologically  continuous 
individuals,  but  only  temporary  chromatic  accumulations  which  are  in- 
constant in  number  and  lose  their  identity  in  the  telophase.  Delia  Valle'fi 
interpretation  of  chromosome  formation  has  been  criticized  by  a  number 
of  writers  and  his  position  shown  to  be  untenable  by  Montgomery  (1910  . 
McClung  (1917),  and  Parmenter  (1919). 

Some  of  the  experiments  on  echinoderm  eggs  with  which  Boveri  (1895, 
1902,  1903,  19046,  1905,  1907)  and  others  supported  the  theory  of  chromo- 
some individuality  may  be  briefly  reviewed. 

Boveri  found  that  if  the  number  of  chromosomes  is  increased  or  de- 
creased by  artificial  means  the  altered  number  appears  at  every  mitosis 
thereafter,     (a)  An  enucleate  egg  fragment  may  be  entered  by  a  sperma- 
tozoon, and  may  then  develop  into  a  larva  with  half  the  •normal  number 
of  chromosomes  in  every  cell,     (b)  In  another  experiment  the  unferl  ilized 
egg  of  a  sea  urchin  was  caused  to  undergo  division  by  artificial  means, 
after  which  a  spermatozoon  was  allowed  to  enter  one  of  the  blastomeres 
(daughter  cells).     A  larva  resulted  in  which  one-half  of  the  cells  had  regu- 
larly 18  chromosomes  (half  the  normal  number)  while  the  other  half  had 
the  normal  36.     (c)  Two  spermatozoa  occasionally  fertilized  one  egg: 
the  cells  of  the  resulting  larvae  had  54  chromosomes,  the  triploid  number. 
Abnormal  mitotic  figures  were  often  formed   in  such  dispermic  -up- 
bringing about  an  irregular  distribution  of  the  chromosomes.     For  ex- 
ample, a  quadripolar  spindle  was  produced,  separal  ingtheo  1  split  chromo- 
somes (108  daughter  chromosomes)  into  four  groups,  with  18,  22,  32,  and 
36   chromosomes  respectively  (Fig.  127  bis).      The  resulting  abnormal 
larva  ("pluteus")  showed  these  four  chromosome  numbers  in  the  cells 
of  four  different  regions  of  its  body.     Boveri  (J91  [)  later  suggested  thai 
malignant  tumors  might  be  due  to  such  abnormal  chromosome  distri- 
bution,    (d)  The  number  of  chromosomes  was  doubled  by  shaking  the 
eggs  while   the  chromosomes  were  split  during  the  early  stages  of  cell- 
division.     In  this  manner  larvae  were  produced  with  72  chromosomes,  the 


164  INTRODUCTION  TO  CYTOLOGY 

tetraploid  number,  in  all  of  their  cells,  (e)  In  the  threadworm,  Ascaris 
megalocephala,  fertilization  of  an  egg  of  the  variety  bivalens  (two  chromo- 
somes) by  a  spermatozoon  of  the  variety  univalens  (one  chromosome) 
resulted  in  a  larva  with  three  chromosomes  in  all  its  cells,  the  chromosome 
contributed  by  the  male  parent  being  distinguishable  from  the  other  two 
(Boveri  1888a;  Herla  1893;  Zoja  1895). 

Results  such  as  the  above  led  Boveri  to  the  conclusion  that  the  number 
of  chromosomes  arising  from  the  reticulum  in  prophase  is  directly  and 
exclusively  dependent  upon  the  number  that  went  to  make  it  up  in  the 
preceding  telophase.  If  a  nucleus  is  reconstructed  in  the  telophase  by  an 
abnormal  number  of  chromosomes  as  the  result  of  a  disturbance  of  the 


*  I  I  ii  ?♦*••♦♦« 


B 


«**4 


Fig.  57. — The  chromosome  complement  of  Hesperotettix  viridis. 
A,  the  12  bivalent  chromosomes  of  the  spermatocyte,  including  the  accessory  chromo- 
some (No.  4.)      B,  complement  from  another  individual,  showing  two  "multiple  chromo- 
somes."    Nos.  11  and  12  have  united  temporarily,  as  have  also  Nos.  4  and  9.      X  1800. 
(After  McClunrj,  1917.) 

mitotic  process,  the  altered  number  invariably  appears  in  the  succeeding 
prophase:  if  extra  chromosomes  are  present  they  are  not  eliminated  in 
any  way  during  the  resting  stages,  and  if  chromosomes  have  been  lost 
during  abnormal  mitosis  they  are  not  replaced.  These  conclusions  have 
been  strikingly  confirmed  by  Sakamura's  (1920)  work  on  cells  subjected 
to  the  influence  of  chloral  hydrate  and  other  agencies  causing  aberrant 
chromosome  behavior. 

Variations  in  Number. — Although  the  number  of  chromosomes  in  a 
given  species  is  on  the  whole  remarkably  constant,  departures  from  nor- 
mal numbers  are  occasionally  observed.  Strasburger  (1905)  believed 
that  the  number,  though  determined  by  heredity,  is  not  so  rigidly  fixed 
that  all  variation  in  the  vegetative  cells  is  excluded;  only  in  the  reproduc- 
tive cells  did  he  hold  constancy  in  number  to  be  necessary.     Much  light 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY       165 

has  been  recently  thrown  upon  such  apparenl  variations  in  Dumber  by 
McClung  (1917)  and  Miss  Holt  (1017)  in  their  researches  on  multiple 
chromosomes  and  chromosome  complexes. 

McClung  finds  in  his  analysis  of  the  chromosome  groups  of  the 
orthopterans  Hesperotettix  and  Mermiria  thai  temporary  associations 
often  occur  between  various  members  of  a  group,  wit  h  t  he  resull  ing  forma- 
tion of  "multiple  chromosomes"  and  a  consequent  decrease  in  theappar 
ent  number.  In  Hesperotettix,  for  instance,  the  cells  normally  have  12 
pairs  of  chromosomes,  but  because  of  the  formation  of  such  multiple 
chromosomes  individuals  with  apparently  11,  10,  or  9  pairs  arc  frequently 
found  (Fig.  57).  For  a  given  individual  the  number  so  formed  is  exactly 
constant,  since  the  members  of  a  multiple  remain  together  in  all  the  cells 
of  the  body;  but  for  the  species  it  is  variable  within  certain  limits,  owing 
to  the  varying  numbers  of  chromosomes  which  may  become  involved  in 
such  multiple  combinations.  In  all  cases  the  full  number  of  chromosome 
pairs  is  present,  but  some  of  them  are  so  combined  that  there  is  an  appar- 
ent, though  not  actual,  variation  in  the  number.  A  similar  condition  Is 
found  in  other  forms  by  Robertson  (1916). 

In  Culex  there  are  three  pairs  of  chromosomes  in  the  somatic  cells. 
During  a  certain  stage  in  the  insect's  metamorphosis  it  has  been  shown 
by  Miss  Holt  (1917)  that  the  chromosomes  may  split  repeatedly,  giving 
cells  with  much  larger  numbers — up  to  72  in  some  cases.  These  Larger 
numbers,  however,  are  nearly  always  multiples  of  three,  indicating  that 
the  subdivision  of  the  chromosomes  is  an  orderly  process.  The  daughter 
chromosomes,  moreover,  that  are  formed  by  the  subdivision  of  each  of 
the  original  six,  remain  more  or  less  closely  associated  as  a  "multiple 
complex,"  which  behaves  as  a  single  individual  in  mitosis.  It  therefore 
appears  that  the  three  pairs  of  chromosomes  "are  made  up  of  <|iin<' 
distinct  individuals  differing  from  each  other  to  such  a  degree  that 
chromatin  split  from  one  cannot  associate  itself  with  that  from  another 
pair.  .  .  .  Chromosome  individuality,  alone,  can  account  for  th< 
conditions." 

Somewhat  similar  evidence  has  been  brought  forward  by  Hance  I  1!»17. 
1918a6).  Hance  finds  that  the  chromosome  Dumber  in  the  spermatogo- 
nia of  the  pig  is  regularly  40,  wdiereas  in  the  somatic  cells  it  varies  from 
40  to  57.  Similarly  in  (Enothera  scintillans,  which  ha-  1">  chromosomes 
in  its  microsporocytes,  there  may  be  from  15  to  21  chromosomes  in  the 
somatic  cells.  Measurements  of  the  members  of  t  he  various  chromosome 
groups  show  that  the  larger  numbers  are  due  to  a  fragmentation,  prob- 
ably of  the  larger  chromosomes,  in  the  somatic  cells.  Such  fragments 
divide  normally,  and  it  appears  probable  that  the  fragments  of  a  single 
original  chromosome  are  held  together  by  colorless  portions  and  behave 
as  a  unit,  much  as  do  the  mult  iple  complexes  of  ( ' ul<  x. 

Sakamura  (1920)  believes  that  the  chief  reason  for  frequently  reported 


166  INTRODUCTION  TO  CYTOLOGY 

inconstancies  in  chromosome  number  is  to  be  found  in  the  chromosome 
constrictions,  which  under  certain  conditions  become  especially  pro- 
nounced and  temporarily  divide  one  or  more  of  the  chromosomes  of  the 
group  into  loosely  connected  smaller  parts.  This  suggestion,  which 
Sakamura  supports  with  much  direct  evidence,  is  probably  one  of  the 
most  fruitful  which  has  been  made  in  this  connection. 

The  theory  of  chromosome  individuality  is  believed  by  McClung 
and  Hance  to  be  strengthened,  rather  than  weakened,  by  such  instances 
of  numerical  variation  as  those  described  above.  McClung  emphasizes 
the  point  that  the  composition  of  a  given  chromosome  can  be  fully  under- 
stood only  if  something  is  known  of  its  genetic  history,  for  what  appears 
as  a  chromosome  may  often  be  either  an  aggregation  of  two  or  three 
chromosomes,  or,  on  the  other  hand,  only  a  portion  of  the  true  chromo- 
some individual.  How  widely  this  interpretation  may  be  applicable  to 
other  reported  cases  of  numerical  variation  and  to  chromosome  structure 
in  general  cannot  at  present  be  stated,  but  it  promises  to  lead  to  signifi- 
cant results. 

Discussion  and  Conclusions. — The  author's  views  on  the  subject  of 
the  individuality  of  the  chromosomes  can  be  most  effectively  stated  in 
the  words  of  McClung  (1917): 

1 .  .  .  the  practical  matter  before  us  is  to  decide  whether  the  metaphase  chromo- 
somes of  two  cells  are  individually  identical  organic  members  of  a  series  because 
they  were  produced  by  the  observed  reproduction  of  a  similar  series  of  the  parent 
cell,  or  whether  the  resemblance  is  independent  of  this  genetic  relation  and  due  to 
chance  association  of  indifferent  materials,  or  to  a  reconstituting  action  of  the  cell 
as  a  whole." 

"If  it  were  possible  for  chromosomes  to  reproduce  themselves  and  still  pre- 
serve their  physical  configuration  unchanged,  there  would  probably  be  little 
question  of  their  continuity  and  individuality — the  demonstration  would  be  self- 
evident.  But  it  happens  that  the  necessities  of  the  case  require  that  each  newly 
produced  chromosome  should  take  part  in  the  formation  of  a  new  nucleus,  through 
whose  activities  the  cell  as  a  whole  and  each  chromosome,  individually,  is  enabled 
to  restore  the  volume  diminished  by  the  act  of  division.  During  this  process  the 
outlines  of  the  chromosomes  become  materially  changed  and  in  their  extreme 
diffusion  can  no  longer  be  traced  in  many  cases.  Because  of  our  limitations  in 
observational  power  the}r  appear  to  be  lost  as  separate  individuals  and  we  are 
thus  deprived  of  the  simple  test  of  observed  continuity.  Later,  in  the  same  cell, 
there  reappears  a  series  of  chromosomes  severally  like  those  which  seemed  to 
disappear  during  the  period  of  metabolic  activity.  We  confront  two  alternative 
explanations  for  this  reintegration  of  the  chromosomes;  either  they  actually 
persist  as  discrete  units  of  extremely  variable  form,  or  they  are  entirely  lost  as 
individual  entities  and  are  reconstituted  by  some  extrinsic  agency.  There  is  no 
other  possible  explanation  and  we  must  weigh  the  facts  for  one  or  the  other  of  the 
alternatives. 

All  the  facts  which  indicate  order  and  system  in  chromosome  features  speak 
for  the  former,  those  which  demonstrate  variability  and  indefiniteness,  for  the 


SOMATIC  MITOSIS  AND  CHROMOSOME  INDIVIDUALITY       167 

latter.     The  case  for  discontinuity  is  strongest  in  tin-  absence  of  any  chromosome 
order,  and  becomes  progressively  weaker  with  <!n'  establishment  of  definite] 
and  precision  in  form  and  behavior." 

"So  far  as  I  can  see  there  is  no  half  way  ground  Del  ween  I  he  assumpl  ion  thai 
the  chromosomes  are  definite;,  self-perpetuating  organic  structures  and  the  other 
which  presents  them  as  mere  incidental  products  of  cellular  action.  According 
to  one  view  individual  chromosomes  are  descendents  of  like  element-  and  pos» 
certain  qualities  and  behavior  because  of  their  material  de-cent,  the  visible 
mechanism  for  which  is  the  process  of  mitosis:  according  to  the  other  any  similari- 
ties that  may  exist  in  the  complexes  are  the  result  of  chance  aggregations  of  non- 
specific materials.  It  is  a  choice  between  organization  and  non-organization  in 
the  last  analysis,  at  least  in  terms  of  cellular  structure-.  To  attempt  the  sub- 
stitution of  a  conception  of  molecular  organization,  which  is  beyond  the 
perience  of  the  biologist  and  which  exceeds  the  present  powers  of  the  chemist  bo 
analyse,  is  to  cast  aside  all  hope  of  solving  the  problem  of  cellular  action,  because 
it  is  necessary  to  understand,  not  only  the  physical  and  chemical  phenomena 
involved,  but  also  their  different  forms  in  the  various  parts  of  the  cell." 

"That  the  chromosomes  do  not  maintain  a  compact  and  easily  recognizable 
form  in  the  interval  between  mitoses  is  accepted  by  many  .  .  .  biologists  as  proof 
that  they  no  longer  exist  as  entities.  All  the  other  manifold  indication-  of  char- 
acter and  continuity  do  not  weigh  against  this  apparent  loss  of  identity.  Doubt- 
less it  would  be  more  satisfying  if  we  could  at  all  times  perceive  the  chromosomes 
in  unchanging  form  in  all  stages  of  cellular  activity,  but  why  we  should  demand 
this  condition  as  a  test  of  individuality  in  the  chromosomes  when  we  unhesitat- 
ingly admit  the  unity  of  the  organism  in  all  the  varied  changes  of  its  develop- 
ment from  a  single  cell,  through  such  complexities  of  change  and  metamorphosis 
as  to  give  rise  to  doubts  of  even  the  phyletic  position  of  some  stages,  it  is  difficult 
to  see.  Being  organic,  the  chromosomes  must  change  their  form,  the}'  must  suffer 
division  of  their  substance  and  they  are  obliged  to  restore  this  loss  through  meta- 
bolic changes.  Since  these  changes  of  substance  take  place  at  surface  contacts 
there  is  an  obvious  advantage  in  increased  superficies  and,  in  common  with  other, 
larger  structural  elements,  the  chromosomes  become  extended  and  their  sub- 
stances are  diffused.  In  this  state  their  boundaries  may  not  be  well  defined  and 
this  circumstance  has  been  seized  upon  as  a  disproof  of  their  continuity." 

''Since  it  is  not  possible  to  observe  directly  the  action  of  the  chromosome  we 
are  obliged  to  make  use  of  indirect  evidence,  seeking  parallels  between  elements 
of  structure  and  action  in  the  chromosomes,  and  the  mass  effect  of  cellular  action 
as  exhibited  in  the  so-called  body  characters.  Such  a  method  is  justified  by  all 
other  experience  in  tracing  relations  between  structure  and  function  in  organisms, 
and  while  it  apparently  resolves  the  organism  into  parts  n\  greater  or  Less  in- 
dependence, has  given  us  our  best  conceptions  of  it  as  a  whole." 

"What  is  postulated  ...  is  that  the  chromosomes  are  self-perpetuating 
entities  with  individual  peculiarities  of  form  and  function  to  identify  them. 
Characteristics  of  form  and  behavior  we  see;  certain  very  definite  parallels  be- 
tween these  and  the  manifestations  of  somatic  characters  exist  beyond  question; 
provision  for  the  perpetuation  of  the  organic  unity  of  the  individual  chromosomes 
is  found  in  the  process  of  mitosis;  the  actual  direct  result  of  its  operation  appear- 
in  the  uniform  conditions  of  the  complex  in  the  individual  animal;  the  extension 


168  INTRODUCTION  TO  CYTOLOGY 

of  this  beyond  the  organism  to  the  group  and  the  means  for  it  in  the  phenomena 
of  maturation  and  fertilization  are  easily  established  by  observation;  the  age 
old  existence  of  all  these  circumstances  is  revealed  by  the  near  approach  to 
uniformity  in  the  chromosome  complex  of  the  multitude  of  species  of  unnumbered 
individuals  constituting  a  family.  And  yet,  in  the  face  of  this  overwhelming 
mass  of  evidence  indicative  of  order,  system  and  specific  chromosome  organiza- 
tion, some  conceive  only  the  action  of  ordinary  chemical  forces,  or  the  chance 
association  of  indifferent  substances,  while  others,  over  impressed  with  the 
thought  of  a  general  coordinating  force  in  the  organism,  deny  significance  to  the 
orderly  play  of  its  cellular  parts." 

"It  is  my  belief  that  the  observed  act  of  reproduction,  by  which  the  organiza- 
tion of  the  chromosomes  is  materially  transmitted  in  each  mitosis,  together 
with  all  facts  indicating  extensive  distribution  of  given  conditions,  definiteness 
of  organization,  uniformity  of  behavior  and  consistence  of  deviation  from  the 
normal,  are  so  many  clear  indications  of  the  individual  character  of  the  chromo- 
somes. Transmutation  of  form,  even  to  an  extreme  degree,  can  not  be  held  as  a 
valid  argument  against  a  persistent  individuality.  A  consideration  of  the  criteria 
applied  to  larger  organic  aggregates  well  supports  this  view.  Such  objects  are 
said  to  possess  individuality  when  they  exhibit  a  more  or  less  definite  unity  which 
is  persistent  and  characterized  by  peculiarities  of  form  and  function.  Most 
clearly  defined  is  this  individuality  when  it  may  be  perpetuated  through  some 
form  of  reproduction  to  find  expression  in  new  units  of  similar  character.  The 
term  does  not  connote  unchangeability,  and  there  may  be  fusions  with  more  or 
less  loss  of  physical  delimitations,  followed  by  separation,  even  after  exchange 
of  substances.  The  test  of  individuality  is  material  continuity,  but  it  does  not 
necessarily  involve  complete  or  entirely  persistent  contiguity.1  An  organism  may 
bud  off  new  individuals  similar  to  itself,  the  substance  of  its  body  differs  from 
time  to  time,  movements  of  parts  take  place,  fragmentation  occurs,  extreme 
attenuation  or  extension  of  substance  is  found,  even  separation  and  recombina- 
tion of  parts  may  happen  and  yet  the  individual  maintains  itself.  What  it  may 
have  been  in  the  past,  what  its  possibilities  of  future  development  are,  what 
potentialities  of  multiplied  individuality  it  suppresses  do  not  affect  the  reality  of 
its  individuality.  It  is,  as  Huxley  says,  'a  single  thing  of  a  given  kind.'  If 
one  such  thing  divides  into  two,  there  are  two  individuals;  if  two  unite  into  one 
indistinguishably  there  is  a  single  individual;  if  a  fusion  of  two  things  occurs  in 
part,  without  loss  of  physical  configuration,  there  are  still  two  individuals  in 
existence.  Only  when  the  substance  of  one  thing  disappears  or  becomes  in- 
corporated integrally  into  the  organization  of  another  does  its  individuality 
depart. 

If  all  these  variations  of  physical  state  may  occur  in  the  history  of  an  organism 
without  sacrifice  of  individuality,  there  can  be  no  reason  for  urging  them  against 
a  conception  of  the  individuality  of  the  self-perpetuating  chromosomes." 

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CHAPTKR  IX 

THE   ACHROMATIC    FIGURE,    CYTOKINESIS,    AND    THE    CELL 

WALL 

THE  ACHROMATIC  FIGURE 

The  spindle  fibers  and  asters  about  the  centrosomes  (when  these  are 
present)  are  collectively  termed  the  achromatic  figure,  in  contradistinction 
to  the  chromatic  figure,  or  chromosomes.  Compared  with  the  chromo- 
somes the  achromatic  figure  is  relatively  little  understood,  which  makes 
it  a  very  unsatisfactory  subject  for  discussion.  We  shall  first  describe 
the  achromatic  figure  in  its  more  common  forms,  and  after  mentioning 
certain  theories  which  have  been  propounded  to  explain  its  origin  and 
nature  we  shall  briefly  review  a  few  of  the  suggestions  which  have  been 
made  on  the  subject  of  the  mechanism  of  mitosis. 

In  Higher  Plants. — In  somatic  mitosis  in  higher  plants  the  achromatic 
figure  is  devoid  of  centrosomes  and  asters.  Ordinarily  it  arises  and  be- 
haves as  follows:  While  the  prophasic  changes  are  taking  place  within 
the  nucleus  the  first  indications  of  spindle  formation  appear  in  the  cyto- 
plasm in  the  immediate  vicinity  of  the  nucleus.  At  the  two  sides  of  the 
latter,  in  the  general  position  of  the  future  spindle  poles,  there  arc  de- 
veloped two  masses  of  more  or  less  hyaline  material,  usually  called  "kino- 
plasmic  caps."  In  these  two  polar  caps  delicate  fibrils  soon  appear,  as 
if  by  a  process  of  condensation  (Fig.  58,  A,  B).  The  nucleus  commonly 
shrinks  at  this  time,  while  the  fibrous  areas  increase  in  size  and  together 
form  a  more  definitely  spindle-shaped  figure.  After  the  nuclear  mem- 
brane has  shrunken  more  closely  about  the  chromosomes  it  goes  into 
solution  and  the  ingrowing  fibers  attach  themselves  to  the  longitudinally 
split  chromosomes.  In  many  cases  the  membrane  disappears  without 
shrinking,  the  fibers  growing  considerably  in  length  to  reach  the  chromo- 
somes. The  latter  quickly  become  regularly  arranged  in  the  equatorial 
plane  preparatory  to  their  separation  (Chapter  VIII).  The  mitotic 
figure  is  now  established  (Fig.  48).  The  many  fibers  composing  the 
spindle  may  focus  at  a  single  sharp  point  at  each  pole,  or  they  may  end 
indefinitely  without  converging  to  a  point,  forming  in  the  hitter  case  a 
broad-poled  figure  which  in  extreme  cases  may  be  as  wide  at  the  poles 
as  at  the  equator  (Fig.  74,  D).  Some  of  the  fibers  extend  from  the  poles 
to  the  chromosomes,  to  which  they  are  attached,  while  others  pass 
through  from  otic  pole  to  the  other  without   being  so  attached:  these 

17.-. 


176 


INTRODUCTION  TO  CYTOLOGY 


two  sets  of  fibers  are  known  respectively  as  mantle  fibers  and  connecting 
fibers.     The  latter  are  also  collectively  termed  the  central  spindle. 

It  is  during  the  anaphases  and  telophases  that  the  connecting  fibers 
become  most  evident;  in  mitotic  figures  with  many  chromosomes  it  may 
be  impossible  to  see  them  at  metaphase.  At  the  beginning  of  the  telo- 
phase they  may  form  a  bundle  no  greater  in  diameter  than  the  daughter 
chromosome  groups,  but  as  the  daughter  nuclei  reorganize  the  fibers 
commonly  bend  outward  at  the  middle,  forming  a  barrel-shaped  phrag- 
moplast  (Fig.  58,  C)  which  in  plants  usually  continues  to  widen  by  the 
addition  of  new  fibers  until  it  comes  in  contact  with  the  lateral  walls  of 
the  cell. 


V 

A 


/{■':::Li^7l^''-' 


Fig.  58. 

A,  spindle  beginning  to  differentiate  in  kinoplasmic  caps  at  poles  of  nucleus  in  Nephro- 
dium.  (After  Yamanouchi,  1908.)  B,  same  in  Marsilia.  (After  Berghs,  1909.)  C,  D, 
the  origin  of  the  cell  wall  in  Pinus:  C,  connecting  fibers  between  daughter  nuclei  at  telo- 
phase; D,  thickenings  appearing  on  fibers.  E,  the  continued  extension  of  the  cell  wall 
after  the  completion  of  mitosis  in  the  endosperm  of  Physostegia  virginiana.  X  215.  (After 
Sharp,  1911.)  F,  multipolar  stage  of  spindle  development  in  microsporocyte  of  Acer 
Negundo.      X  1125.      (After  Taylor,  1920.) 


While  the  above  changes  are  occurring  the  new  cell  wall  which  is  to 
be  formed  between  the  daughter  nuclei  begins  to  differentiate.  As  the 
central  spindle  widens  the  fibers  become  fainter  near  the  nuclei  and  more 
prominent  at  the  equatorial  region:  this  appearance  seems  to  be  due  to 
the  flow  of  the  material  composing  the  fibers  toward  the  latter  region. 
On  the  thickened  fibers  there  now  appear  small  swellings  (Fig.  58,  D) 
which  increase  in  size  until  they  fuse  to  form  a  continuous  plate  across 
the  equator  of  the  mother  cell,  thus  dividing  the  latter  into  two  daughter 
cells.  As  this  cell  plate  undergoes  further  changes  (see  p.  190)  the  fibers 
disappear  completely,  first  near  the  two  nuclei  and  ultimately  at  the 
equatorial  region  near  the  new  wall.  If  the  cell  undergoing  division 
is  very  broad  it  often  happens  that  wall  formation  begins  near  the  center 
of  the  phragmoplast  while  the  latter  is  still  extending  laterally.     In 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  CELL  WALL     177 

extreme  cases  wall  formation  may  still  be  seen  in  progress  al  the  periphery 
after  the  fibers  have  completely  disappeared  al  the  central  region  (Fig. 
58,  E).  Such  is  notably  the  case  in  the  tangential  divisions  of  elongated 
cambium  cells  (Bailey  1919,  1920). 

The  spindle  in  many  cases  has  an  origin  somewhal  differenl  from  thai 
described  above.  The  first  indication  of  its  differentiation  is  here  the 
appearance  of  a  weft  of  fine  fibrils  in  the  cytoplasm  all  around  the  nucleus 
(Fig.  84,  E).  As  these  fibrils  increase  in  size  and  number  they  may 
form  several  distinct  groups  extending  in  various  directions,  thus  giving 
a  multipolar  spindle  (Fig.  58,  F).  Some  of  the  groups  then  gradually 
disappear,  while  others  alter  their  positions  and  coalesce,  so  thai  a  bipolar 
spindle  eventually  results.  This,  in  general,  is  the  manner  in  which  the 
spindle  arises  in  the  microsporocytes  of  angiosperms.  For  example, 
Lawson  (1898,  1900,  1903)  finds  that  in  Cobcea,  Gladiolus,  and  Ins  a 
zone  of  granular  "perikaryoplasm"  collects  about  the  nucleus  during 
the  prophases  of  mitosis.  When  the  nuclear  membrane  dissolves,  this 
substance  together  with  the  linin  of  the  nucleus  forms  a  fibrous  network 
which  grows  out  into  several  cones  of  fibers,  and  these  later  become 
arranged  in  two  opposed  groups. 

In  Animals. — In  the  majority  of  animal  cells,  and  in  certain  cells  of 
lower  plants  also,  the  achromatic  figure  is  a  much  more  elaborate  struc- 
ture than  that  of  the  higher  plants  described  above.  This  is  due  to  the 
presence  of  centrosomes,  which  with  their  asters  are  very  conspicuous  at 
the  time  of  mitosis.  Commonly  the  aster  is  not  present  during  the  resting 
stages  of  the  cell,  but  cases  are  known  in  which  both  centrosome  and 
aster  are  visible,  forming  with  other  materials  an  "attraction  sphere'"  in 
the  cytoplasm.  As  the  process  of  mitosis  begins  (Fig.  59),  an  aster,  if 
not  already  present,  develops  about  the  centrosome.  The  centrosome 
divides,  and  as  the  daughter  centrosomes  move  apart  each  is  seen  to  be 
surrounded  by  its  own  aster,  and  a  small  group  of  fibers  ("central  spin- 
dle") extends  between  them.  The  achromatic  figure,  made  up  of  the 
asters  and  the  spindle  connecting  them,  is  known  both  at  this  stage  and 
later  as  the  amphiaster.  As  the  daughter  centrosomes  continue  to  sepa- 
rate the  astral  rays  increase  in  prominence.  Some  of  the  rays  grow  into 
the  nucleus  when  its  membrane  disappears  and  become  attached  as 
mantle  fibers  to  the  chromosomes,  while  the  lengthening  central  spindle 
between  the  asters  becomes  the  cent  ral  spindle  port  ion  <  connect  ing  fibei  - 
of  the  completed  mitotic  figure  (Fig.  49).  All  the  fibers  focus  upon  the 
centrosomes. 

During  the  anaphase  the  asters  remain  very  conspicuous,  but  as  the 
telophases  progress  they  gradually  fade  from  view,  except  in  those  forms 
which  have  a  more  or  less  permanent  attraction  sphere.  Iside  from  the 
presence  of  centrosomes  and  asters  the  achromatic  figure  in  animal  cells 
differs  most  conspicuously  from  that  of  higher  plant  cells  in  its  behavior 

12 


178 


INTRODUCTION  TO  CYTOLOGY 


Fig.    59. — Mitosis    in    the    spermatocyte    of    Salamandra. 

I,  prophase,  centrosomes  in  astrosphere  substance;  latter  spread  out  on  nucleus.  77, 
prophase;  bivalent  chromosomes  formed;  centrosomes  beginning  to  diverge;  central  spin- 
dle and  asters  developed.  Ill,  late  prophase:  nuclear  membrane  dissolved;  spindle 
fibers  attaching  to  chromosomes,  centrosomes  moving  apart.  IV,  anaphase:  connecting 
fibers  prominent.  V,  telophase:  constriction  of  cell  nearly  complete;  mid-body  forming 
on  central  spindle  or  interzonal  fibers.      (After  Meves,  1907.) 


)()(). 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  CELL  WALL     179 

during  the  telophases.  Instead  of  forming  thickenings  which  become  an 
equatorial  cell  plate,  the  connecting  fibers  play  relatively  little  part  in 
cytokinesis.  One  or  more  granules  may  be  differentiated  on  the  fibers 
at  the  equatorial  region,  forming  the  so-called  "mid-body/'  but  the 
actual  division  of  the  cell  is  brought  about  by  the  development  of  a 
cleavage  furrow,  as  will  be  described  in  the  section  on  cytokinesis. 

Intranuclear  Figures.— In  the  above  described  cases  of  mitosis  in 
plants  and  animals  the  achromatic  figure  is  derived  mainly  from  the  cyto- 
plasmic region  of  the  cell,  the  nuclear  materials  playing  a  relatively 
minor  part.  In  a  number  of  forms,  both  among  animals  and  plants  (fungi, 
for  example),  the  spindle  arises  entirely  in  the  nuclear  region,  forming 
an  intranuclear  figure  which  may  be  completely  established  before  the 


■ 


I 

■ 


■  I  ' 


■ 


mix 


' 


'  ■        c 

Fig.  61. 
A,  B,  anaphase  and  telophase  of  mitosis  in  ascus  of  Laboulbenia  chcetophuni.      X  1350. 
(After  Faull,   1912.)      (See  also  Fig.  22.)      C,  intranuclear  mitotic  figure  in  oogonium  of 
Fucus.      (After  Yamanouchi,  1909.) 


nuclear  membrane  disappears.  Cases  are  known  in  which  the  centro- 
somes  themselves  are  also  intranuclear,  but  usually  these  bodies  lie  in 
the  cytoplasm  against  the  nuclear  membrane,  so  that  although  the  spindle 
portion  of  the  figure  is  within  the  nucleus  the  asters  lie  in  the  cytoplasm. 
In  the  division  of  the  nucleus  in  the  ascus  of  an  ascomycete,1  to  take 
a  single  example,  the  process  is  as  follows  (Figs.  22;  01  A,  B):  The 
centrosome,  which  in  ascomycetes  is  often  discoid  in  shape,  lies  against 
the  nuclear  membrane.  As  mitosis  begins  an  aster  develops  in  the  cyto- 
plasm about  the  centrosome,  and  the  latter  divides  to  form  two  daughter 
centrosomes.  The  central  spindle,  if  formed  at  all,  docs  not  persist. 
From  each  of  the  daughter  centrosomes,  which  begin  to  move  apart 
along  the  nuclear  membrane,  a  group  of  fibers  extends  into  the  nucleus 
where  the  chromosomes  are  being  formed  from  the  reticulum.  The 
centrosomes  finally  reach  opposite  sides  of  the  nucleus,  and  their  two 

1  For  references  to  the  literature  of  mitosis  in  ascomycetes  Bee  page  -00. 


180  INTRODUCTION  TO  CYTOLOGY 

groups  of  fibers  become  arranged  in  the  form  of  a  sharp  poled  spindle 
extending  through  the  nucleus  with  the  chromosomes  at  the  equator. 
The  nuclear  membrane  commonly  remains  intact  until  the  chromosomes 
approach  the  poles  at  anaphase;  it  then  disappears,  allowing  the 
nucleolus,  which  has  remained  unchanged,  to  escape  into  the  cytoplasm 
nearby.  Between  the  two  densely  packed  daughter  chromosome  groups 
there  extends  a  long  strand  of  chromatic  material:  this  soon  disappears 
and  the  two  daughter  chromosome  groups  reorganize  two  daughter 
nuclei  not  separated  by  a  wall.  In  those  cases  in  which  the  division  of  the 
fungus  nucleus  is  followed  by  the  development  of  a  separating  wall  the 
latter  is  formed  by  a  cleavage  furrow  independently  of  the  achromatic 
figure. 

Origin  of  the  Figure. — Having  before  us  the  above  examples  of  the 
achromatic  figure,  we  may  now  refer  very  briefly  to  some  of  the  ideas 
which  have  been  advanced  regarding  the  details  of  its  origin  in  the  cell.1 

Early  observers  looked  upon  the  whole  mitotic  figure — chromosomes, 
spindle,  and  all — as  a  transformed  nucleus,  all  the  structures  being  formed 
from  the  nuclear  material  at  each  mitosis.  Strasburger,  who  first  held 
this  view,  later  (1888),  with  Hermann  (1891),  believed  the  spindle  to  arise 
wholly  from  the  cytoplasm,  whereas  0.  Hertwig  pointed  out  cases  in 
which  the  astral  rays  arise  from  the  cytoplasm  and  the  spindle  from  the 
linin  reticulum  of  the  nucleus.  Flemming  (1891)  derived  the  fibers  from 
the  linin  and  the  nuclear  membrane.  It  soon  became  evident  that  the 
spindle,  although  in  some  cases  arising  entirely  within  the  nucleus  or 
wholly  from  the  cytoplasm,  is  commonly  made  up  of  materials  derived 
from  both  regions,  as  is  evident  from  the  examples  described  in  the  fore- 
going paragraphs. 

When  van  Beneden  and  Boveri  announced  their  view  that  the  centro- 
some  is  a  permanent  cell  organ,  transmitted  by  division  to  daughter  cells 
and  directly  concerned  in  the  formation  of  the  asters,  the  theory  was 
adopted  that  the  figure  arises  from  the  cytoplasm  as  a  result  of  the 
influence  of  the  centrosome.  The  centrosome  therefore  came  to  be 
known  as  "the  dynamic  center  of  the  cell."  Although  this  organ  does 
play  a  conspicuous  role  when  present,  its  importance  in  connection  with 
the  achromatic  figure  was  somewhat  diminished  when  it  became  evi- 
dent that  many  centrosomes  do  not  persist  from  one  cell  generation 
to  the  next,  and  that  such  bodies  are  entirely  absent  from  the  cells  of 
higher  plants. 

Rearrangement  Theories. — Many  attempts  have  been  made  to  account 
for  the  formation  of  the  achromatic  fibrils  in  the  cytoplasm.  According 
to  some  the  fibers  and  astral  rays  arise  as  the  result  of  a  morphological 
rearrangement  of  the  preexistent  protoplasmic  structure,  chiefly  under 

1  Extensive  reviews  of  the  early  theories  are  given  by  Wilson  (1900,  pp.  72-86  and 
316-329) 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  CELL  WALL     181 

the  influence  of  the  centre-some.  Butschli  (1876),  who  looked  upon 
protoplasm  as  alveolar  in  nature,  held  thai  the  rays  are  nol  really  fibers, 
but  only  the  lamellae  between  radially  elongated  alveolae  aboul  the  cen- 
trosome.  It  was  the  opinion  of  Wilson  (1899)  on  the  other  hand,  thai 
the  rays  are  actual  fibers,  though  their  material  is  derived  from  the 
alveolar  walls.  Klein  (1878)  and  others  who  believed  protoplasm  to  be 
ultimately  fibrillar  or  reticular  in  structure,  regarded  the  rays  as  radially 
arranged  fibrillae.  Van  Beneden  (1883)  supposed  these  fibrillae  to  be 
derived  partly  from  the  intranuclear  reticulum,  and  Rabl  I  L889) 
pointed  out  that  they  are  continuous  with  the  unaltered  cytoplasmic 
mesh  work  and  arise  by  a  direct  transformation  of  the  latter.  In  Passi- 
flora  Williams  (1899)  found  that  the  nuclear  membrane  forms  a  meshwork 
connecting  the  linin  reticulum  with  the  cytoplasmic  reticulum,  all  three 
together  organizing  the  spindle. 

Special  Substance  Theories. — According  to  another  group  of  theories 
the  spindle  and  asters  are  not  formed  merely  by  the  rearrangement  of  a 
structure  already  present,  but  arise  from  a  special  substance  in  the  cell. 
This  substance  was  held  by  some  to  be  a  constantly  present  constituent 
of  the  cell,  forming  the  achromatic  figure  at  the  time  of  mitosis  and  re- 
maining in  reserve  through  the .  resting  stages.  Boveri's  archoplasm 
hypothesis  in  its  earlier  form  (1888)  was  a  prominent  development  of 
this  idea.  According  to  this  hypothesis  the  attraction  sphere  is  composed 
of  a  distinct  substance  called  archoplasm,  which  consists  in  turn  of  fine 
granules  or  microsomes  aggregated  about  the  centrosome  as  a  result  of 
the  centrosome's  attractive  force.  The  entire  achromatic  figure  was 
held  to  arise  from  this  mass  of  archoplasm,  the  fibers  and  astral  rays 
growing  out  from  it  like  roots,  to  be  withdrawn  again  into  the  daughter 
masses  of  archoplasm  at  the  two  poles  during  the  closing  phases  of  mitosis. 
In  this  way  each  daughter  cell  was  thought  to  receive  half  of  the  archo- 
plasm. Although  other  workers  (Watase  1894)  also  held  that  the  fibers 
are  outgrowths  of  the  centrosome  or  centrosphere  substance,  it  was  made 
evident  later  that  the  material  composing  the  fiber  conies  from  the  cyto- 
plasm, being  added  to  the  growing  fiber  at  its  end.  This  was  the  view  of 
Druner  (1894,  1895).  Boveri  later  (1895)  modified  his  archoplasm  hypo- 
thesis, adopting  the  view  that  the  fiber  is  formed  from  the  substance  of 
the  cytoplasm  and  not  necessarily  from  a  constantly  present  archoplasm. 

Another  theory  based  on  the  idea  of  a  special  substance  in  the  cell 
was  that  of  Strasburger  (1892,  1897,  1898).  Strasburger  held  that  the 
cell  has  two  kinds  of  protoplasm:  an  active  fibrillar  kinoplasm  and  a  less 
active  alveolar  trophoplasm.  The  former consl  it  utes  t  heectoplasl .  cent  ro- 
somes,  the  mitotic  fibers,  and  the  contractile  substance  of  cilia  and 
allied  structures.  The  kinoplasm  is  thus  concerned  with  the  motor 
work  of  the  cell,  whereas  the  trophoplasm  has  to  do  chiefly  with 
nutrition. 


182  INTRODUCTION  TO  CYTOLOGY 

The  nucleolus  has  been  thought  by  some  observers  to  furnish  material 
for  the  formation  of  the  spindle,  because  of  the  fact  that  it  very  commonly 
disappears  from  view  at  about  the  time  the  spindle  begins  to  differentiate. 
It  is  possible  that  in  some  cases  there  may  be  a  connection  of  this  sort 
between  nucleolus  and  spindle,  but  it  is  clear  that  this  cannot  serve  as  a 
general  interpretation  of  spindle  origin. 

That  the  achromatic  figure  may  arise  from  a  special  substance  not 
constantly  present  in  the  cell,  but  formed  anew  at  each  mitosis,  is  a 
theory  which  several  workers  have  advanced.  The  researches  of  Devise 
(1914)  and  Miss  Nothnagel  (1916)  may  be  cited  for  illustration.  Devise, 
as  the  result  of  a  careful  study  of  the  development  of  the  spindle  in  the 
microsporocytes  of  Larix,  concluded  that  the  spindle  is  not  formed  by  the 
rearrangement  of  any  preexistent  nuclear  or  cytoplasmic  structures,  but 
arises  from  a  substance  which  develops  in  the  nuclear  region  during  the 
late  prophases  (after  diakinesis).  He  was  not  able  to  decide  whether  this 
substance  is  of  purely  nuclear  origin  or  is  formed  when  the  karyolymph 
comes  in  contact  with  the  cytoplasm.  The  interaction  of  karyolymph  and 
cytoplasm  is  emphasized  by  Miss  Nothnagel  in  her  work  on  Allium. 
She  points  out  that  the  contact  of  newly  formed  karyolymph  with  the 
cytoplasm  at  telophase  brings  about  the  precipitation  of  the  nuclear 
membrane,  and  that  in  an  analogous  manner  an  exosmosis  of  karyo- 
lymph through  the  nuclear  membrane  into  the  cytoplasm  during  prophase 
causes  the  precipitation  of  fine  fibrils  around  the  nucleus,  these  fibrils  then 
developing  into  the  spindle.  The  achromatic  figure  therefore  arises 
from  a  special  substance,  but  this  substance,  as  in  the  case  of  Larix, 
is  newly  formed  at  each  mitosis. 

Conclusion. — In  general  it  may  be  said  that  although  the  spindle 
fibers  and  the  motor  and  contractile  elements  of  the  cell  appear  to  have 
a  substantial  relationship  with  one  another,  the  substance  common  to 
them  is  probably  "not  to  be  regarded  as  being  necessarily  a  permanent 
constituent  of  the  cell,  but  only  as  a  phase,  more  or  less  persistent,  in 
the  general  metabolic  transformation  of  the  cell  substance"  (Wilson). 
Indeed  the  conspicuous  tendency  on  the  part  of  cytologists  at  present 
is  to  regard  the  achromatic  figure  neither  as  a  mere  rearrangement  of  a 
structure  previously  present,  nor  as  a  form  assumed  by  a  special  spindle 
substance,  but  rather  as  the  result  of  streaming,  gelation,  and  other 
temporary  alterations  in  the  colloidal  substratum.  This  interpretation 
is  strongly  supported  by  the  microdissection  studies  to  be  cited  in  a 
subsequent  paragraph. 

The  Mechanism  of  Mitosis.— Since  the  phenomenon  of  mitosis  was 
first  described  there  have  been  put  forward  a  number  of  theories  to  ac- 
count for  the  operation  of  the  achromatic  structures  in  bringing  about  the 
separation  of  the  daughter  chromosomes  and  for  the  division  of  the  cell. 
Many  of  the  suggestions  undoubtedly  contain  elements  of  truth,  but  it 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  CELL  WALL     183 

must  be  admitted  that  there  is  no  immediate  prospect  of  a  satisfactory 

solution  of  these  problems. 

Contractility. — One  of  the  simplest  and  most  widely  accepted  theories 
was  that  of  fibrillar  contractility  suggested  by  Klein  (1878)  and  van 
Beneden  (1883,  1887),  according  to  which  the  chromosomes  arc  simply 
dragged  apart  by  the  contraction  of  two  opposed  groups  of  Bpindle  fibers. 
This  theory  and  its  modifications  are  fully  reviewed  by  Wilson:  i1  will 
be  sufficient  here  to  point  out  that,  whereas  many  facts  were  cited  in  its 
favor,  and  elastic  models  made  which  simulated  the  supposed  contraction 
and  its  results  (Heidenhain),  the  further  evidence  brought  forward  by 
Hermann  (1891),  Druner  (1894,  1895),  Calkins  (1898),  and  others  Led  to 
the  general  restriction  of  the  role  of  contractility,  until  it  became  appa- 
rent that  this  factor,  although  it  may  contribute  to  the  general  result, 
must  be  of  minor  importance.  The  contractility  factor  appeared  again 
in  the  more  elaborate  theory  proposed  by  Rhumbler,  which  may  be 
briefly  stated  as  follows:  The  centrosome  arises  as  a  local  solidification 
of  the  walls  of  the  alveolae;  the  denser  constituents  of  the  protoplasm 
collect  at  this  point  and  form  an  attraction  sphere,  driving  the  less  dei 
constituents  to  the  other  parts  of  the  cell  where  the  pressure  is  lower;  this 
migration  of  fluid  affects  particularly  those  strands  of  the  protoplasmic 
reticulum  which  radiate  more  directly  from  the  centrosomes;  th< 
strands  or  rays,  in  giving  up  their  fluid,  shorten,  and  thus  exert  a  trac- 
tive force  which  draws  the  daughter  chromosomes  apart.  In  this  theory. 
therefore,  the  main  factors  are  streaming  and  contractility. 

Streaming.— The  phenomena  of  streaming  and  surface  tension  have 
been  prominent  factors  in  several  attempts  to  explain  both  karyokinesis 
and  cytokinesis.  The  role  of  streaming  in  karyokinesis  has  been  held  to 
be  especially  important  since  Butschli,  "Hertwig,  and  Fol  showed  many 
years  ago  that  currents  exist  in  the  protoplasm.  Rhumbler  (1896,  1 899  . 
Morgan  (1899),  Wilson  (1901),  and  Conklin  (1902)  all  held  that  the 
astral  rays  are  due  at  least  in  part  to  centripetal  currents.  This  inter- 
pretation has  recently  been  confirmed  by  Chambers  (1917)  in  his  micro- 
dissection studies  on  the  living  cell.  With  regard  to  the  aster  ( Jhambers 
says:  "The  formation  of  the  aster  consists  in  the  gelation  of  the  hyalo- 
plasm which  comes  under  the  influence  of  the  astral  center.  A  hyaline 
liquid  separates  out  during  the  gelation  and  flows  in  innumerable  centri- 
petal paths  toward  the  center  where  it  accumulates  to  form  a  sphere. 
This  centripetal  flow  brings  about  an  arrangement  of  the  gelled  hyalo- 
plasm containing  the  cell-granules  into  radial  strands  separated  by  the 
hyaline-liquid  paths.  This  produces  the  astral  figure.  The  strands  of 
gelatinized  cytoplasm  merge  peripherally  into  the  surrounding  liquid 
cytoplasm  or  reach  and  anchor  themselves  in  the  substance  of  the  gelled 
surface  when  the  aster  is  fully  formed.  The  liquid  rays  merge  centrally 
into  the  substance  of  the  sphere,  the  liquid  of  the  rays  and  of  the  sphere 
being  thus  identical." 


184  INTRODUCTION  TO  CYTOLOGY 

Sakamura  (1920),  although  holding  the  fibers  to  be  important  agents 
in  the  normal  separation  of  the  daughter  chromosomes,  observes  that  in 
abnormal  nuclear  divisions  where  no  fibers  are  present  the  chromosomes 
still  show  movements  which  are  probably  due  to  streaming  of  the  cyto- 
plasm and  to  surface  tension  phenomena. 

The  relation  of  streaming  and  surface  tension  to  cytokinesis  will  be 
discussed  in  the  section  dealing  more  particularly  with  cytokinesis. 

Osmosis. — In  a  theory  of  the  mechanics  of  karyokinesis  proposed  by 
Lawson  (1911)  the  principal  factor  involved  is  osmosis.  Lawson's  ex- 
planation is  essentially  as  follows.  During  the  late  prophase  karyolymph 
passes  outward  through  the  nuclear  membrane  by  osmosis,  this  loss  of 
fluid  resulting  in  a  contraction  of  the  nucleus.  Owing  to  the  fact  that 
the  cytoplasmic  reticulum  is  continuous  with  the  nuclear  membrane  this 
contraction  sets  up  radial  lines  of  tension  in  this  reticulum  on  all  sides  of 
the  nucleus.  As  the  process  continues  these  lines  or  " fibers"  gradually 
become  arranged  in  two  opposed  groups,  while  the  nuclear  membrane  to 
which  they  are  attached  continues  to  contract  until  it  actually  enwraps 
each  double  chromosome.  To  each  double  chromosome  there  are  thus 
attached  fibers  which  represent  stretched  and  distorted  regions  of  the 
cytoplasmic  reticulum  extending  to  the  two  sides  of  the  cell.  When  the 
chromosomes  become  properly  arranged  at  the  equatorial  plane  the 
fibers,  which  are  under  considerable  tension,  are  able  to  pull  the  daughter 
chromosomes  apart  and  draw  them  to  the  poles.  As  the  fibers  relax  they 
resume  their  true  reticular  state.  Although  the  chromosomes  are  thus 
drawn  apart  by  the  shortening  of  " fibers"  attached  to  them,  Lawson 
points  out  that  this  is  not  to  be  regarded  as  a  case  of  true  active  contrac- 
tility, but  only  as  a  release  of  tension  set  up  in  the  passive  but  elastic 
cytoplasmic  reticulum  as  the  result  of  the  exosmosis  of  karyolymph  from 
the  nucleus.  This  theory  has  been  severely  criticized  by  a  number  of 
writers,  chiefly  on  the  grounds  that  such  an  enwrapping  of  the  chromo- 
somes by  the  nuclear  membrane  as  Lawson  describes  cannot  be  demon- 
strated in  many  objects  subsequently  examined,  and  that  the  membrane 
frequently  goes  into  solution  when  both  it  and  the  growing  fibers  are 
still  some  distance  from  the  chromosomes. 

Electrical  Theories. — The  striking  resemblance  between  the  achromatic 
figure  and  the  lines  of  force  in  an  electromagnetic  field  early  led  to  at- 
tempts to  account  for  mitosis  on  the  basis  of  electrical  principles.  Several 
investigators,  working  with  various  chemical  substances,  succeeded  in 
modelling  fields  of  force  that  illustrated  graphically  the  changes  supposed 
to  take  place  in  the  dividing  cell.  In  later  years  the  electromagnetic 
interpretation  was  again  brought  into  prominence  by  Gallardo,  Hartog, 
and  Prenant.  At  first  Gallardo  (1896)  believed  the  two  spindle  poles 
to  be  of  unlike  sign,  but  later  (1906),  as  the  result  of  the  researches  of 
Lillie  (1903)  (see  p.  62)  on  the  behavior  of  nucleus  and  cytoplasm  in 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS.  AND  CELL  WALL      L85 

the  electromagnetic  field,  he  concluded  thai  the  chromosomes  and  the 
cytoplasm  carry  charges  of  unlike  sign:  the  daughter  centrosomes  repel 
each  other  and  move  apart  because  of  their  like  sign,  the  spindle  pol 
being  of  like  sign  also.  The  movement  of  the  chromosomes  to  the  pol< 
he  held  to  be  due  to  the  combined  action  of  two  forces:  the  mutual  repul- 
sion of  the  similarly  charged  daughter  chromosomes,  and  the  attraction 
between  the  oppositely  charged  centrosomes  and  chromosomes. 

The  fact  that  the  two  centrosomes  and  hence  the  two  spindle  poles 
are  electrically  homopolar  (Lillie)  and  alike  osmotically  at  once  makes  it 
apparent  that  the  mitotic  figure  does  not  represent  an  ordinary  electro- 
magnetic field,  for  in  the  latter  the  poles  are  of  unlike  sign  the  field  is 
heteropolar.  It  has  consequently  been  suggested  by  Prenant  (1910)  and 
Hartog  (1905,  1914)  that  the  mitotic  figure  is  the  seat  of  a  special  force, 
analogous  to  electrostatic  force  but  not  identical  with  it ,  which  is  peculiar 
to  living  organisms.     This  new  force  they  call  "mitokinetism." 

A  large  amount  of  discussion  has  centered  about  the  possible  r61e  of 
electrical  forces  in  mitosis,  and  many  kinds  of  normal  and  abnormal 
mitotic  phenomena  have  been  cited  as  evidence  for  various  views.  So 
far  as  conclusive  statements  are  concerned,  there  is  disappointingly  little 
of  a  definite  nature  that  can  be  said.  Meek  (1913)  asserts  that  the  only 
generalization  which  is  at  present  possible  is  the  negative  one  that 
"the  mitotic  spindle  is  not  a  figure  formed  entirely  by  the  action  of 
forces  at  its  poles." 

Conclusion. — In   conclusion   we   may  emphasize   the   fact    thai    the 
achromatic  figure  depends  for  its  operation  upon  a  variety  of  interacting 
factors.     Certain  investigators  have  doubtless  done  good  service  in  em- 
phasizing the  importance  of  one  or  another  of  these  factors — streaming, 
surface  tension,   contractility,  gelation,  electrical  phenomena,   and  the 
like — but  it  has  become  increasingly  evident  that  in  no  one  of  them  alone 
is  the  key  to  the  problem  of  mitosis  to  be  found.     In  spite  of  the  confi- 
dence that  some  progress  has  been  made,  at  least  in  the  elucidation  01 
certain  phenomena  which  must  have  a  part  in  any  ultimate  explanation, 
it  is  nevertheless  true  that  the  statements  made  twenty  years  ago  by 
Wilson  (1900,  p.  Ill)  may  be  taken  as  an  essentially  accurate  expression 
of  the  condition  of  the  subject:  "When  all  is  said,  we  must  admit  that 
the  mechanism  of  mitosis  in  every  phase  still  awaits  adequate  physio- 
logical analysis.     The  suggestive  experiments  of  Butschli  and  Heidenhain 
lead  us  to  hope  that  a  partial  solution  of  the  problem  may  be  reached 
along  the  lines  of  physical  and  chemical  experiment.     At  present  we  can 
only  admit  that  none  of  the  conclusions  thus  far  reached,  whether  by 
observation  or  by  experiment,  are  more  than  the  first  naifH  attempts  to 
analyse  a  group  of  most  complex  phenomena  of  which  we  have  little  real 
understanding." 


186 


INTRODUCTION  TO  CYTOLOGY 


CYTOKINESIS 

In  the  foregoing  pages  discussion  has  been  limited  largely  to  karyo- 
kinesis.  In  the  present  section  attention  will  be  directed  to  cytokinesis, 
or  the  division  of  the  extra-nuclear  portion  of  the  cell. 

In  plants  the  wall  separating  the  two  daughter  cells  is  formed  by 
two  general  methods:  cell  plate  formation  and  furrowing.  The  first  and 
more  common  of  these  methods,  by  which  a  wall  is  formed  in  close 
association  with  the  spindle  fibers  at  the  close  of  mitosis,  has  been  briefly 
described  in  the  foregoing  section  on  the  achromatic  figure  (p.  176)  and 
will  be  taken  up  in  greater  detail  in  the  following  section  on  the  cell  wall 
(p.  190).  At  this  point  we  shall  therefore  describe  the  second  method, 
that  of  furrowing,  which  in  plants  is  seen  most  conspicuously  in  the 
thallophytes  and  in  the  microsporocytes  of  the  higher  plants.  The 
review  of  the  subject  given  by  Farr  (1916)  will  be  followed. 

Thallophytes.— In  Spirogyra  Strasburger  (1875)  showed  that  the 
wall  between  the  two  daughter  cells  appears  as  a  " girdle"  or  ring-like 
ingrowth  from  the  side  wall  of  the  parent  cell.     This  wall  continues  to 


--' ff 


figi 


Fig.  63. 
Only  the  central  part  of  the  cell 


Fig.  62. 
Fig.   62. — Cytokinesis  by  furrowing  in    Closterium. 
is  shown.      X  700.      (After  Lutman,  1911.) 

Fig.    63. 
A,  Cleavage  furrows  beginning  to  form  at  periphery  of  sporangium  of  Rhizopus  nigricans. 
X  1500.     B,  Cleavage  in  the  sporangium  of  Phycomyces  nitens:  intersporal  substance  in  the 
angular  furrows.      X  500.      (Both  after  D.  B.  Swingle,  1903.) 

grow  centripetally  by  the  addition  of  new  material  at  its  inner  edge  while 
the  protoplast  develops  a  deep  cleavage  furrow,  the  process  continuing 
until  the  separating  wall  is  completed  at  the  center  of  the  cell.  A 
somewhat  similar  process  occurs  in  Closterium  (Lutman  1911)  (Fig.  62). 
In  the  brown  algae  Sphacelaria  (Strasburger  1892;  W.  T.  Swingle  1897) 
and  Dictyota   (Mottier   1900)   the  wali  develops  uniformly  across  the 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AM)  (ELL  WALL     1ST 


whole  equatorial  plane  at  the  same  time,  and  not  as   i  progressive  in- 
growth from  the  periphery. 

In  the  fungi  Harper  and  others  showed  that  the  two  daughter  cells  are 
separated  by  the  development  of  a  cleavage  furrow  in  which  the  new  wall 
is  laid  down.  In  large  multinucleate  masses  that  become  broken  up  into 
spores  this  progressive  cleavage  is  a  very  complicated  process.  The 
manner  in  which  the  furrows  develop  is  shown  in  the  studies  of  Timberlake 
(1902)  on  Hydrodictyon,  D.  B.  Swingle  (1903)  and  Moreau  (1913)  on 
Rhizopus  and  Phycomyces,  Davis  (1903)  on  Saprolegnia,  Kvtz  (1907 
on  Synchytrium,  and  Harper  (1899,  1900,  1914)  on  Synch \yt num.  Pilobol 
Sporodinia,  Fuligo,  and  Didymium.  In  Rhizopus  (Fig.  63,  A)  the 
cleavage  furrows  begin  to  form  both  at  the  peripheral  membrane  of  the 
sporangium  and  at  the  columella  and  work  gradually  into  the  multi- 
nucleate protoplasm,  eventually  cutting  out  multinucleate  blocks  which 
become  the  spores.  In  Phycomyces  (Fig.  63,  B)  small  vacuoles  appear  in 
the  midst  of  the  multinucleate  protoplasm,  enlarge  and  become  stellate, 
and  cut  out  spore  masses  with  from  1  to  12  nuclei 
each.  In  the  myxomycete,  Fuligo,  the  cleavage 
is  from  the  surface  inward,  and  the  multinucleate 
blocks  are  subdivided  by  further  furrowing  into 
uninucleate  spores.  In  Didymium  the  spores  are 
delimited  in  a  similar  way  by  furrows  which 
begin  to  form  along  the  young  capillitium  fila- 
ments in  the  interior  of  the  multinucleate  mass 
as  well  as  at  its  periphery. 

Microsporocytes. — In  the  microsporocytes  of 
the  higher  plants  it  has  been  shown  with  great 
clearness  by  Farr  (1916,  1918)  that  the  quadri- 
partition  to  form  spore  tetrads  of  the  tetrahedral 
type  is  brought  about  by  furrowing,  previous  ac- 
counts having  generally  stated  that  the  walls 
are  formed  by  the  cell  plate  method.  Farr  finds 
that  after  the  four  microspore  nuclei  are  formed 
they  all  become  connected  by  a  series  of  six 
spindles,  or  sets  of  connecting  fibers.  The  two 
spindles  of  the  second  maturation  mitosis  may 
persist,  four  new  ones  being  added,  or  the  two 
may  disappear,  six  new  ones  being  developed. 
Although  some  sporadic  thickenings  may  ap- 
pear on  these  fibers  they  have  nothing  to  do  with  the  formation  of  the 
separating  walls,  there  being  no  eentrifugally  growing  cell  plates  such  as 
are  seen  in  cells  dividing  by  the  cell  plate  method.  Constriction  fur- 
rows appear  at  the  periphery  of  the  cell  (Fig.  64)  andgTOW  inward  until 
they  meet  at  the  center,  dividing  the  protoplast  simultaneously  into  four 


Fn..    64.     t  \  tokinesis  by 
furrowing     in     the     micro- 
sporoc}  t<-      of       Vicol 
■    l  100.    {After  Farr,  1916. 


188  INTRODUCTION  TO  CYTOLOGY 

spores.  Any  fibers  which  these  furrows  encounter  as  they  grow  inward 
are  probably  incorporated  in  the  new  wall,  but  they  play  no  prominent 
part  in  wall  formation:  the  development  of  the  furrows  appears  to  be 
entirely  independent  of  the  fibers  present. 

In  his  first  paper  (1916)  Farr  states  that  the  microspore  tetrads  of 
the  bilateral  type  are  usually  formed  by  the  cell  plate  method,  a  wall 
being  formed  across  the  diameter  of  the  microsporocyte  on  the  connecting 
fibers  after  the  first  maturation  mitosis,  and  the  two  daughter  cells 
being  divided  in  a  similar  way  after  the  second  mitosis.  In  his  second 
contribution  (1918)  he  shows  that  in  Magnolia  such  tetrads  also  are 
formed  by  furrowing.  After  the  first  mitosis  a  cleavage  furrow  starts 
to  form,  but  its  development  is  arrested  until  after  the  second  mitosis, 
when  it  resumes  its  growth  toward  the  center  and  forms  a  wall  across  the 
diameter  of  the  spherical  protoplast.  At  the  same  time  other  new 
furrows  subdivide  each  hemisphere,  so  that  four  uninucleate  microspores 
result.  Farr  states  that  no  case  of  bipartition  by  furrowing  is  known  in 
the  higher  plants;  bipartition  begins  in  Magnolia,  but  the  furrow  ceases  to 
grow  until  other  furrows  are  formed  after  the  second  mitosis,  the  eventual 
division  occurring  by  quadripartition.  In  the  lower  plants,  however, 
bilateral  tetrads  may  be  formed  by  the  cell  plate  method.  It  is  the  opinion 
of  Farr  that  furrowing  in  microsporocytes  is  due  to  conditions  similar  to 
those  which  bring  it  about  in  animal  eggs  (see  below),  since  both  float 
freely  in  a  liquid. 

Animals. — In  animals  there  is  found  nothing  corresponding  to  the 
formation  of  a  cell  plate  on  the  spindle  fibers  and  its  development  into  a 
thick  wall  such  as  is  seen  in  plants.  As  noted  in  the  section  on  the 
achromatic  figure,  there  is  often  a  slight  differentiation  at  this  region 
(the  "mid-body"),  but  it  has  nothing  to  do  with  cytokinesis,  which  is 
brought  about  by  simple  constriction  or  furrowing.  This  process  is 
most  easily  followed  in  the  segmenting  egg.  In  small  eggs,  such  as  those 
of  worms,  the  daughter  cells  (blastomeres)  round  up  and  become  more  or 
less  spherical,  whereas  in  larger  eggs,  such  as  that  of  the  frog,  a  cleavage 
furrow  appears  at  one  pole  and  develops  through  the  egg  without  altering 
the  shape  of  the  latter,  so  that  the  first  two  blastomeres  have  the  form  of 
hemispheres.  It  is  with  animal  eggs  that  most  of  the  researches  on  the 
mechanism  of  cytokinesis  by  furrowing  have  been  carried  out. 

Mechanism  of  Furrowing.— Attempts  to  explain  furrowing  and  the 
separation  of  the  daughter  cells  on  physico-chemical  grounds  have  been 
rather  numerous.  Many  years  ago  Butschli  (1876)  advanced  the  view 
that  as  a  result  of  a  specific  activity  on  the  part  of  the  centrosomes  cyto- 
plasmic currents  are  set  up  which  flow  toward  the  centrosomes  and 
produce  a  higher  surface  tension  at  the  equator  of  the  cell,  this  in  turn 
bringing  about  furrowing  and  cell-division.  McClendon  (1910,  1913) 
also  reported  an  increase  in  surface  tension  at  the  region  of  furrowing. 


THE  ACHROMATIC  FIClTRE,  CYTOKINESIS,  AND  CELL  WALL     ISO 

On  the  contrary,  Robertson  (1911,  1913)  and  others  all  riliute  furrowing 
rather  to  a  decrease  in  the  equatorial  surface  tension,  this  decrease  being 
due  to  a  diffusion  of  materials  toward  that  region  from  the  daughter 
nuclei.  Evidence  favoring  Biitschli's  interpretation  has  been  afforded 
by  the  studies  of  Spek  (1918).  Spck  imitated  furrowing  and  division 
with  oil  and  mercury  droplets  in  water,  and  showed  thai  by  Lowering  the 
surface  tension  at  two  poles  of  the  droplet  the  relatively  higher  surface 
tension  at  the  equatorial  region  could  be  made  to  bring  about  the  con- 
striction and  fission  of  the  droplet.  In  both  droplet  and  dividing  egg 
he  found  streamings  such  as  Erlangen  (1897)  had  described  in  the  nema- 
tode egg:  an  axial  movement  polewards  to  the  regions  of  low  surface 
tension  and  a  superficial  streaming  toward  the  equatorial  region  of  higher 
surface  tension,  the  streams  turning  inward  at  the  furrow   (  Fig.  65  . 


Fig.  65. — Diagram  showing  streaming  and  furrowing  in  the  onix  of  Rhabditis 
(A)  and  an  oil  droplet  (B).     (After  Spek,  1918.) 

Although  the  causes  of  the  initial  changes  in  surface  tension  in  the  case 
of  the  cell  are  relatively  obscure,  these  experiments  of  Spek  show  beyond 
question  that  alteration  in  surface  tension  and  streaming  are  very  im- 
portant factors  in  cell-division  of  this  type. 

The  relation  of  periodic  changes  in  the  viscosity  of  the  egg  substance 
to  cytokinesis  by  furrowing  has  recently  been  discussed  by  Chamb 
(1919).  Immediately  after  the  entrance  of  the  spermatozoon  into  the 
echinoderm  egg  the  sperm  aster  begins  to  differentiate  as  a  semi-solid 
region  near  the  sperm  head.  (See  p.  279.)  When  the  aster  is  most  fully 
developed  the  egg  has  its  maximum  viscosity  (Heilbrunn  1915  A- 
the  aster  disappears  the  egg  again  becomes  more  fluid.  Then  a  second 
solidification  begins  at  two  centers  forming  the  amphiaster,  or  bipolar 
figure.  The  growth  of  these  two  semi-solid  masses  results  in  the  elonga- 
tion of  the  egg,  and  eventually  in  the  development  of  a  cleavage  furrow 
in  the  more  fluid  portion  of  the  egg  substance  separating  them.  After 
cleavage  is  complete  the  semi-solid  masses  (asters)  revert  to  a  more  fluid 
state.  The  formation  of  the  cleavage  furrow,  moreover,  may  be  pre- 
vented by  mechanical  means.  At  the  second  mitosis  in  eggs  so  treated 
(binucleate  eggs)  there  are  four  centers  of  semi-solidification  rather  than 
two,  and  the  egg  cleaves  simultaneously  into  four  blastomeres.  An  egg 
cut  into  two  pieces  during  the  amphiaster  stage  will,  provided  it  does  not 


190  INTRODUCTION  TO  CYTOLOGY 

return  to  the  fluid  state,  continue  to  cleave  along  the  normal  plane 
through  the  equator  of  the  cell  as  if  nothing  unusual  had  happened.  All 
of  these  observations  indicate  a  close  dependence  of  cytokinesis  upon  the 
temporary  differentiation  of  semi-solid  masses  in  the  egg  cytoplasm,  and 
throw  much  light  upon  the  question  of  the  true  nature  of  the  achromatic 
figure. 

THE  CELL  WALL 
Probably  the  most  striking  difference  which  meets  the  eye  in  a  com- 
parison of  animal  and  plant  tissues  lies  in  the  relative  degree  of  dis- 
tinctness with  which  the  limits  of  the  individual  cells  may  be  made  out. 
Animal  cells  as  a  rule  are  separated  only  by  very  thin  limiting  membranes 
which  in  many  tissues  are  so  delicate  as  to  be  scarcely  discernible, 
whereas  the  cells  of  plants  usually  possess  conspicuous  firm  walls,  which 
in  the  case  of  woody  plants  become  greatly  thickened  and  afford 
mechanical  support  to  large  bodies. 

The  Primary  Wall  Layer. — Since  the  time  when  mitotic  cell-division 
was  first  carefully  studied  with  the  aid  of  modern  methods  it  has  been 
known  that  in  the  cell  wall  of  plants  the  primary  layer,  or  middle  lamella 
(the  "intercellular  substance"  and  " cement"  of  early  writers),  is  formed 
in  most  cases  in  close  connection  with  the  spindle  fibers  at  the  close  of 
mitosis.1     The  exact  manner  of  its  origin,  however,  has  proved  to  be  a 
very  difficult  point  to  determine,  and  has  formed  the  subject  of  a  long 
continued   controversy.     (See   papers   of  Timberlake   and   Allen,    1900 
and   1901.)     During  the  telophases  of  mitosis  the  spindle  fibers  con- 
necting the  two  daughter  nuclei  develop  thickenings  (Fig.  58,  D),  enlarge 
until  they  come  in  contact  with  one  another  and  fuse  to  form  a  cell  plate, 
or  partition,  between  the  daughter  cells.     For  some  time  it  was  thought 
(Strasburger  1875,  1882,  1884)  that  the  cell  plate  so  formed  became  at 
once  the  middle  lamella,  upon  which  secondary  and  frequently  tertiary 
layers  were  subsequently  deposited  by  the  protoplasts  on  either  side. 
Strasburger  here  found  support  for  his  theory  that  the  cell  wall  is  essen- 
tially a  transformed  layer  of  the  protoplast,  in  opposition  to  Nageli  and 
von   Mohl,  who  regarded  it  as  primarily  a  secretion  product.     As  a 
result  of  further  researches,  however,  he  later  (1898)  abandoned  this 
view  and  adopted  an  interpretation  that  had  been  suggested  by  Treub 
(1878),  namely,  that  the  cell  plate  formed  by  the  consolidation  of  the 
swellings  (" microsomes")  on  the  spindle  fibers  very  soon  splits  to  form 
the  plasma  membranes  of  the  two  daughter  cells,  and  that  there  is  then 
secreted   between   these   membranes   by   the   protoplasts   a   substance 
which  becomes  the  primary  layer,  or  middle  lamella.     The  correctness 
of  this  view  was  confirmed  by  the  careful  researches  of  Timberlake 
(1900)  and  Allen  (1901).     Timberlake  pointed  out  that  in  the  micro- 

1  Discussion  is  here  limited  to  the  walls  of  higher  plant  tissues.     The  ectoplast  of 
naked  cells  has  been  dealt  with  in  Chapter  III. 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  CELL  WALL     191 


sporocytes  of  Larix  and  the  root  cells  of  Allium  the  connecting  fibers 
first  thicken  near  the  nuclei,  then  become  uniform  throughout  their 
length,  and  finally  become  swollen  at  the  equatorial  region,  indicating  a 
transfer  toward  that  region  of  the  material  that  is  to  compose  the  cell 
plate.  Allen  was  able  to  show  not  only  thai  the  middle  lamella  itself 
may  increase  in  thickness  by  the  addition  of  new  material  before  tin 
secondary  layers  begin  to  be  laid  down,  but  also 
that  it  consists  in  reality  of  two  layers  representing 
the  secretions  contributed  by  the  two  daughter 
protoplasts.  Where  these  two  masses  of  secreted 
material  meet  there  is  developed  a  median  plane  of 
weakness  which  is  ordinarily  invisible  but  along 
which  the  lamella  invariably  splits  when  inter- 
cellular spaces  are  developed  by  the  rounding  up 
of  the  cells.  By  the  use  of  proper  staining  methods 
it  has  been  found  possible  to  differentiate  this 
"primary  cleavage  plane."  The  continuity  of  the 
middle  lamella  is  interrupted,  if  at  all,  only  by 
the  fine  pores  through  which  pass  the  protoplasmic 
strands  connecting  adjacent  cells.     (See  p.  46.) 

Secondary  and  Tertiary  Wall  Layers  (Fig.  66). — 
It  is  probable  that  the  deposition  of  the  secondary 
layer  begins  after  the  cell  has  reached  nearly  or 
quite  its  full  size,  though  to  this  there  are  ap- 
parently certain  exceptions.  The  secondary  layer, 
which  seems  to  be  formed  with  considerable 
rapidity,  differs  from  the  primary  layer  not  only 
chemically  (see  below)  but  also  in  structure,  being 
interrupted  by  circular  or  elongated  areas  in 
which  no  secondary  substance  is  deposited,  so 
that  the  cells  at  these  places  are  separated  only  by 
the  delicate  primary  membrane.  Such  a  wall  is 
said  to  be  "pitted,"  the  primary  lamella  extending 
across  the  pit  being  termed  the  closing  membrane. 
The  central  portion  of  this  membrane  sometimes 
(vascular  cells  of  gymnosperms  chiefly)  has  a  more  or  leg  conspicuous 
thickening  known  as  the  torus.  The  portion  of  the  membrane 
around  the  torus  is  pierced  by  fine  pores:  in  some  cases  these  may 
become  so  large  and  numerous  that  the  torus  appears  to  be  suspended 
on  a  meshwork  (Fig.  67),  while  extreme  cases  are  known  in  which  it  is 
held  in  place  only  by  a  few  strands.  In  bordered  pits  (Fig.  68  the  second- 
ary wall  overarches  the  margins  of  the  closing  membrane.  In  this  type 
of  pit,  characteristic  chiefly  of  water-conducting  cells  of  the  gymnosperms, 
the  closing  membrane  is  of  such  a  nature  thai  it-  position  in  the  center  of 


p  « 


Fig.  r>6. — Longitudinal 
and  transverse  Bections 
of  a  gymnosperm  tra- 
cheid;  />.  primary  wall 
or  middle  lamella;  8, 
secondary  layer;  /.  spiral 
tertiary  thickening. 


192 


INTRODUCTION  TO  CYTOLOGY 


the  pit  is  readily  altered.  Probably  because  of  changes  in  pressure  it 
swings  to  the  side  of  the  pit;  the  torus  then  lies  against  the  pit  opening, 
or  "mouth,"  and  the  pit  is  blocked  except  for  slow  diffusion  through  the 
rather  thick  torus.  The  latter  may  even  be  forced  tightly  into  the  pit 
mouth. 

The  secondary  wall  layer  may  be  even  more  limited  in  extent,  only  a 
small  portion  of  the  primary  wall  being  covered.  Such  is  the  case  in 
protoxylem  cells,  in  which  the  secondary  layer  is  deposited  in  the  form  of 
rings  and  spirals  (Fig.  4).     This  form  of  thickening,  together  with  the 


Fig.  67.  Fig.  68. 

Fig.  67. — Pits  in  the  wood  of  Larix,  showing  perforations  in  pit  membrane.  X  800. 
{After  Bailey.) 

Fig.   68. — Diagram  of  bordered  pit  of  coniferous  wood. 

A,  section  of  pit  showing  closing  membrane  supporting  the  torus,  and  secondary  layers 
on  each  side  of  middle  lamella.  B,  face  view  of  same.  C,  section  showing  torus  forced 
against  mouth  of  pit.      (After  Bailey.) 

peculiarly  extensible  character  of  their  primary  walls,  allows  for  the  great 
increase  in  length  of  these  cells  necessitated  by  the  continued  growth  of 
the  young  organs  in  which  they  chiefly  function.  In  some  cells,  notably 
the  tracheids  of  certain  gymnosperms  and  the  vessels  of  many  angio- 
sperms,  a  tertiary  layer  is  deposited  upon  the  secondary  wall.  This  ter- 
tiary layer  takes  the  form  of  slender  spirals,  rings,  and  other  figures 
resembling  the  secondary  thickenings  of  protoxylem  cells. 

The  Physical  Nature  of  the  Cell  Wall.— Hugo  von  Mohl  (1853,  1858) 
first  expressed  the  idea  that  the  cell  wall  grows  by  apposition,  i.e.,  by  the 
deposition  of  material  in  successive  laminae.  Although  certain  other 
workers  (Wigand  1856)  supported  this  view,  it  became  over-shadowed  for 
a  time  by  the  theory  of  Nageli.  This  investigator,  as  a  result  of  his  classic 
researches  on  the  wall  and  on  starch  grains  (1858,  1862,  1863),  concluded 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  (ELL  WALL     193 

that  the  wall  is  made  up  of  ultra  microscopic  crystalline  micella  sur- 
rounded by  water  films.  Growth  of  the  wall  in  thickness  and  in  area  he 
believed  to  be  due  to  the  intercalation  of  new  micellae  between  theoldones, 
a  process  termed  intussusception.  Contrasted  with  this  was  Si  rasburger's 
development  of  the  apposition  theory  (1882,  1889).  Although  Stras- 
burger  agreed  that  the  wall  had  both  solid  and  liquid  constituents,  he 
held  that  the  latter  were  not  complex  micell®,  but  only  molecules  linked 
together  in  the  form  of  a  reticular  framework  by  their  chemical  affinities. 
Growth  in  area  he  thought  was  merely  a  matter  of  stretching  without  t  he 
intercalation  of  additional  particles,  while  increase  in  thickness  was 
supposed  to  be  accomplished  by  apposition,  or  the  deposition  of  layers 
of  new  material  in  the  form  of  small  particles,  or  microsomes.  The 
striatums  which  both  he  and  von  Mohl  observed  in  the  wall  substance 
were  regarded  by  Strasburger  as  due  to  the  linear  arrangement  of  thi 
microsomes. 

That  the  cell  wall  is  not  merely  a  lifeless  secretion  of  the  protoplast, 
but  contains  protoplasm  in  some  form,  is  a  view  which  has  often  been 
upheld,  and  involves  problems  which  are  still  far  from  being  solve,]. 
Prominence  was  given  to  the  view  by  Wiesner  (1886),  who  looked  upon 
the  growing  cell  membrane  as  a  living  part  of  the  cell.  Following  Stras- 
burger's  early  view,  he  held  the  primary  layer  to  be  wholly  protoplasmic. 
and  supposed  the  growing  wall  to  be  made  up  of  regularly  arranged 
particles,  which  he  called  dermatosomes,  connected  by  fine  fibrils  of 
protoplasm.  Growth  was  accomplished  by  the  intussusception  of  new 
dermatosomes.  Evidence  in  support  of  Wiesner's  interpretation  was 
brought  forward  by  Molisch  (1888),  who  showed  that  when  tyloses  come 
into  contact  pits  are  formed  exactly  opposite  each  other  in  the  two 
abutting  walls,  a  phenomenon  which  it  would  be  difficult  to  explain  were 
the  walls  without  living  substance. 

The  new  intussusception  theory  of  Wiesner  was  accepted  by  a  Dumber 
of  workers  including  Haberlandt  and  Zacharias  (1891).  The  apposition, 
or  lamination,  theory  of  Strasburger  also  had  many  supporters,  among 
them  being  Noll  (1887),  Klebs  (1886),  Zimmermann  (1887),  and  Askena 
(1890).  According  to  Pfeffer  (1892)  both  processes,  the  intussusception 
of  new  particles  or  molecules  and  the  apposition  of  new  material  in  layers, 
are  concerned  in  the  development  of  the  wall.  This  view  was  later 
adopted  by  Strasburger  (1898),  and  has  received  general  acceptance. 
But  much  work  must  be  done  before  any  final  conclusion  can  be  drawn  re- 
garding many  points.  Especially  obscure  is  the  exact  relationship  of 
the  protoplasm  and  the  wall.  The  solution  of  this  difficull  problem 
must  await  the  results  of  further  inquiries  by  both  the  cytologisl  and  the 
biochemist. 

The  Chemical  Nature  of  the  Cell  Wall.- -Through  the  researches  of 
Payen  (1842),  Fremy  (1859),  Kabsch  (1863),  Wiesner  I  1864,  1878),  and 

13 


194  INTRODUCTION  TO  CYTOLOGY 

particularly  Mangin  (1888-1893)  it  has  been  found  that  the  chief  constitu- 
ents of  the  newly  formed  cell  walls  of  plants  are  pectose  and  cellulose — 
that  the  primary  wall  or  middle  lamella  consists  of  pectose,  the  secondary 
layer  of  pectose  and  cellulose,  and  the  tertiary  layer  of  cellulose.  These 
substances  however,  rarely  exist  in  the  wall  in  pure  and  unmodified  form. 
The  pectose  of  the  primary  layer  changes  later  to  insoluble  pectates, 
especially  the  pectate  of  calcium,  while  the  secondary  and  tertiary  layers 
very  soon  become  greatly  changed  in  composition,  not  alone  through  the 
addition  of  a  variety  of  new  substances,  but  also  through  an  actual  trans- 
formation which  in  some  cases  appears  to  be  complete.  For  example,  the 
secondary  and  tertiary  layers  of  xylem  cells,  although  at  first  containing 
much  cellulose,  may  later  become  so  completely  transformed  into  or  re- 
placed by  lignin  that  they  show  no  reaction  whatever  to  cellulose  stains. 
In  some  cases  the  primary  wall  may  undergo  a  certain  amount  of  lignifi- 
cation  also.  The  walls  of  many  cells  become  heavily  impregnated  with 
cutin  or  suberin,  the  latter  substance  being  responsible  for  the  peculiar 
character  of  corky  tissues.  Infiltration  by  cutin,  or  "cutinization,"  is 
to  be  distinguished  from  "cuticularization,"  by  which  is  meant  the  secre- 
tion of  a  layer  of  cutin  (cuticle)  on  the  outside  of  the  cell.  A  variety  of 
mineral  substances,  such  as  silica,  calcium  oxalate,  and  calcium  carbonate, 
as  well  as  more  complex  organic  compounds,  such  as  tannin,  oils,  and 
resins,  are  often  deposited  in  the  walls  of  old  cells.  The  heartwoods 
of  trees  owe  their  qualities  largely  to  the  presence  of  these  additional 
materials. 

In  spite  of  these  modifications,  however,  it  is  still  true  that  cellulose 
is  the  substance  chiefly  characteristic  of  plant  cell  walls  in  general.  Al- 
though cellulose  has  been  identified  in  certain  animals,  the  membranes  of 
practically  all  animal  cells  are  composed  of  other  substances,  such  as 
keratin,  elastin,  gelatin,  and  chitin.  In  the  fungi  also  the  role  of  cellulose 
appears  to  be  played  in  part  by  chitin. 

The  Walls  of  Spores. — Special  attention  has  been  given  to  the  develop- 
ment of  the  elaborate  walls,  or  coats,  of  the  spores  of  various  plants  in  a 
number  of  investigations.  Strasburger  (1882,  1889,  1898,  1907)  con- 
cluded that  such  coats  arise  by  two  general  methods:  (1)  by  the  growth  in 
thickness  (by  apposition)  of  the  original  wall  of  the  spore  cell  through  the 
activity  of  the  protoplast,  as  in  the  pollen  grains  of  Malva  and  other  angio- 
sperms,  and  (2)  by  a  deposition  of  material  upon  the  original  wall  by  the 
tapetal  fluid  in  which  the  young  spores  lie,  as  in  the  case  of  the  megaspore 
of  Marsilia. 

The  highly  specialized  coats  of  the  megaspore  of  Selaginella  have  been 
most  intensively  studied,  particularly  by  Fitting  (1900,  1906)  and  Miss 
Lyon  (1905),  whose  accounts  disagree  in  several  points.  At  the  close  of 
the  tetrad  division  there  is  formed  about  each  young  spore  a  thick  gela- 
tinous ''special  wall,"  at  the  inner  surface  of  which,  according  to  Fitting, 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  A ND  CELL  WALL      1  95 


the  spore  coats  begin  to  differentiate.  The  exospore  first  appears,  and 
just  outside  of  it  the  rough  perispore  soon  begins  to  develop.  Then  a 
second  layer,  the  mesospore,  is  formed  within  the  exospore.  Between  the 
protoplast,  which  is  at  this  time  very  small,  and  the  mesospore,  and 
between  the  exospore  and  the  mesospore,  there  are  developed  t  w<>  cavities 
filled  with  a  sporangial  fluid  which  furnishes  material  to  the  growing 
coats.  Emphasis  is  placed  on  the  fact  that  the  coats  are  able  to  mere; 
in  thickness  while  they  have  no  immediate  contact  with  the  protopls 
The  protoplast  now  expands,  after  which  a  third  coat,  tin-  endospore,  is 
formed  at  its  surface.  The  mature  spore  thus  has  three  coats  according 
to  Fitting's  interpretation,  which  Denke  (1902)  and  Campbell  (1902) 
confirmed. 


>  m      ym     (1°  • 


e  x.  s.rrii      e  n. 

Fig.    69. — The    developing   megaspore    coat   of   Selaginella    rupestris: 

p,  protoplast  with  nucleus;  en,  endospore;  s.m.,    undifferentiated    portion    of  "spore 
membrane;"  ex,  exospore:  the  outer  denser  portion  is  the  "  perinium."         1  ;•■     /,■■>>,  L905. 


Miss  Lyon  found  that  the  spore  coats  (S.  rupestris)  begin  to  differ- 
entiate in  the  midst  of  the  "spore  membrane"  ("special  wall:*  Pittii 
rather  than  at  its  inner  surface  as  Fitting  thought.  The  exospore 
first  appears  as  a  double  zone,  the  outer  pari  of  which  become-  the  per- 
inium  (perispore:  Fitting)  (Fig.  69).  The  small  protoplasi  gradually 
expands  and  pushes  back  the  undifferentiated  inner  portion  of  the  Bpore 
membrane;  and  while  it  does  so  a  second  coal  is  formed  at  its  surface 
and  becomes  the  endospore  (mesospore:  Fitting)  which  increases  in  thick- 
ness by  lamination.  In  another  species  (S.  emiliana)  the  exospore  and 
endospore  form  simultaneously.      Miss  Lyon  thus  finds  two  coats  rather 


196  INTRODUCTION  TO  CYTOLOGY 

than  three,  but  points  out  that  a  portion  of  the  spore  membrane  which 
may  remain  in  an  undifferentiated  condition  until  a  late  stage  may  easily 
be  mistaken  for  a  third  coat.  The  two  " spaces"  in  the  immature  spore 
wall  she  holds  to  be  undifferentiated  regions  in  the  spore  membrane,  and 
not  cavities  filled  with  a  foreign  fluid;  and  further  urges  that  the  proto- 
plast is  at  all  times  in  contact  with  the  gelatinous  spore  membrane  in 
which  the  coats  are  differentiating,  opposing  the  view  that  the  latter 
have  the  power  of  independent  growth  in  thickness. 

Evidence  favoring  the  view  that  the  spore  coats  can  grow  while  not 
in  contact  with  the  protoplast  has  been  brought  forward  by  Beer  (1905, 
1911)  and  Tischler  (1908).  Beer  asserts  that  although  both  the  primary 
wall  and  the  secondary  thickening  layer  of  the  pollen  grain  (in  certain 
members  of  the  Onagraceae)  originate  in  intimate  connection  with  the 
plasma  membrane,  most  of  their  subsequent  growth  occurs  by  intussus- 
ception while  they  are  completely  separated  from  the  protoplast,  which 
secretes  the  material  used.  The  development  of  the  pollen  wall  in 
Ipomoea  purpurea  has  been  described  in  great  detail  by  Beer.  Around 
each  young  spore  immediately  after  its  formation  there  appears  a  tem- 
porary gelatinous  "  special  wall,"  upon  the  inner  surface  of  which  the 
protoplast  deposits  the  exine,  or  outer  spore  coat.  This  is  at  first  homo- 
geneous, but  soon  differentiates  into  a  thin  outer  lamella  and  an  inner 
zone  made  up  of  a  network  of  thickenings  with  the  rudiments  of  spines 
at  its  nodes.  Both  the  spines  and  the  small  rodlets,  which  develop  in  a 
clear  space  appearing  between  the  outer  lamella  and  the  network  of 
thickenings  (mesospore) ,  undergo  most  of  their  development  after  they 
are  separated  from  the  protoplast.  Tischler  (1908)  reports  that  the 
exine  of  the  pollen  of  sterile  Mirabilis  hybrids  may  continue  to  increase 
in  thickness  after  the  protoplast  begins  to  degenerate. 

As  an  example  of  the  formation  of  spore  coats  through  the  activity 
of  a  tapetal  Plasmodium  may  be  taken  the  case  of  Equisetum,  described 
by  Beer  (1909)  and  Hannig  (1911).  The  spores  of  this  form  have  three 
coats:  an  endospore,  an  exospore,  and  a  perispore  consisting  of  several 
layers  including  the  one  which  splits  to  form  the  "elaters."  The  young 
spore  cell  at  first  has  a  simple  membrane,  the  rudiment  of  the  exospore. 
The  walls  of  the  tapetal  cells  dissolve,  allowing  the  cell  contents  to  flow 
freely  among  the  spores  as  a  tapetal  plasmodium.  Upon  the  spore 
membrane  the  plasmodium  deposits  successively  (1)  an  inner  gelatinous 
layer,  (2)  the  "middle  coat,"  (3)  an  outer  gelatinous  layer,  and  (4)  the 
elater  layer.  The  exospore  develops  from  the  original  membrane  after  the 
middle  coat  is  formed,  and  the  endospore,  or  innermost  coat,  is  developed 
last  of  all. 

From  this  brief  review,  to  which  other  examples  might  be  added,  it  is 
evident  that  spore  coats  may  develop  in  a  variety  of  ways,  but  too  little 
is  known  to  warrant  any  statement  as  to  which  method  may  be  the  most 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  CELL  WALL     197 

general  one.  Although  cytological  interest  centers  chiefly  in  other 
problems,  further  studies  on  spore  coats  would  not  only  contribute  to 
our  understanding  of  cell  wall  formation,  but  would  al«>  aid  in  Bolving  the 
problem  of  the  possible  existence  of  protoplasm  in  the  wall. 

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Flemming,   W.     1891.     Neue  Beitrage  zur  Kenntniss  der  Zelle.     II.  Arch.  Mikr. 

Anat.  37:  685-751.     pis.  38-40. 
Fremy,  E.     1859a.     Recherches  chimiques  sur  la  composition  des  cellules  vegetales. 
Comptes  Rend.  Acad.  Sci.     Paris  48:  202-212. 
18596.     Recherches  chimiqies  sur  la  cuticle.     Ibid.  667-673. 
1859c.     Recherches  sur  la  composition  chimique  du  bois.     Ibid.  862-868. 
Gallardo,   A.     1896a,     La  carioquinesis.     Ann.   Soc.    Cientif.     Argentina  42. 
18966.     Essai  d 'interpretation  des  figures  karyokinetiques.     Ann.   Mus.  Nac.  d. 

Buenos  Aires. 
1901.     Les  croisments  des  radiations  polaires  et  l'interpretation  dynamique  des 

figures  de  karyokinese.     Soc.  de  Biol.  53. 
1906.     L'interpretation  bipolaire  de  la  division  karyokinetique.     Ann.  Mus.  Nac. 

d.  Buenos  Aires  13:  259. 
1909.     La  division  de  la  cellule  phenomene  bipolaire  de  caractere  electro-colloidal. 
Arch.  Entw.  28:  125-154.     Figs.  9. 
Hannig,  E.     1911.     Ueber  die  Bedeutung  der  Periplasmodien.     I.  Die  Bildung  des 
Perispors  bei  Equisetum.     II.  Die  Bildung  der  Massulse  von  Azolla.     Flora  102 : 
209-278,  pis.  13,  14.     figs.  17. 
Harper,  R.  A.     1899.     Cell  division  in  sporangia  and  asci.     Ann.  Bot.  13 :  467-525. 
pis.  24-26. 
1900.     Cell  and  nuclear  division  in  Fuligo  varians.     Bot.  Gaz.  30:  217-251.     pi.  14. 
1914.     Cleavage  in  Didymium  melanospermum  (Pers.)  Macbr.     Am.  Jour.  Bot.  1 : 
127-144.     pis.  11,  12. 
Hartog,  M.     1905.     The  dual  force  of  the  dividing  cell.     I.     The  achromatic  spindle 
figure   illustrated   by   magnetic  chains  of  force.     Proc.  Roy.  Soc.     London  76 
B:  548-567.     pis.  9-11. 
1914.     The   true   mechanism   of   mitosis.     Arch.    Entw.   40:    33-64.     figs.  16. 
Heilbrunn,  L.  V.     1915.    Studies  in  artificial  parthenogenesis.     II.     Physical  changes 

in  the  egg  of  Arbacia.     Biol.  Bull.  29:  149-203. 
Hermann,  J.     1891.     Beitrage  zur  Lehre  von  der  Entstehung  der  karyokinetischen 

Spindel.     Arch.  Mikr.  Anat.  37 :  569-586.     pi.  31. 
Jorgensen,  M.     1913.     Zellenstudien.     II.  Die  Ei-  und  Nahrzellen  von  Piscicola. 
Arch.  Zellf.  10:  125-160.     pis.  13-18. 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AND  CELL  WALL     199 

Kabsch,  W.     1863.     UntersuchungeD  iiber  die  chemische  Beschaffenheil  der  Pflan- 
zengewebejmitBezugaufdieneuesten  Arbeiten  Fremya  liber  diesen  GezenafcinH 
Jahrb.  Wiss.  Bot.  3 :  357-399. 

Klebs,  G.     1886.     Ueberdie  Organization  der  Gallerte  bei  einigen  Algen  und  FTagel- 

laten.     Unters.  Bot.  Inst.  Tubingen  2 :  333-418.     pis.  3,  l 
Klein,  E.     1878,  1879.     Observations  on  the  structure  of  cells  and  nuclei      Quar 

Jour.  Micr.  Sci.  18:  315-339,  pi.  16;  19:  125-175.     pi.  7. 
Kuhn,  A.     1920.     Untersuchungen  zur  kausalen  Analyse  der  ZeUteilun*     I      irch 

f.  Entw.  46:  259-327.     pis.  11,  12.     figs.  21. 
Lawson,  A.  A.     1898.     Some  observations  on  the  developmeni  of  the  karyokinetic 

spindle  in  the  pollen  mother  cell  of  Cobcea  scandens.     Proc   ( lalif    lead    Sci    I',  t 

111  1:  169-184.     pis.  33-36. 

1900.     Origin  of  the  cones  of  the  multipolar  spindle  in  Gladiolus      Bol    Gaz    -JO  • 
145-153.     pi.  12. 

1903.     Studies  in  spindle  formation,     ibid.  36:  81-100.     pis.  15    16. 
1911.     Nuclear  osmosis  as  a  factor  in  mitosis.     Trans.  Rov    S„-    Fdinb    48-   1 
137-161.     pis.  1-4. 
Lillie,  R.  S.     1903.     On  differences  in  the  direction  of  the  electrical  connection  of 

certain  free  cells  and  nuclei.     Am.  Jour.  Physiol.  8:  273-283. 
Lutman,    B.   F.     1911.     Cell  and  nuclear  division  in   Closterium      Bot     Gaz      51- 

401-430.     pis.  22,  23. 
Lyon,  F.     1905.     The  spore  coats  of  Seiaginella.     Bot.  Gaz.  40 :  285-295.     pis.  10,  11. 
Mangin,  L.     1888.     Sur  la  constitution  de  la  membrane  des  vegeHaux.     ("our 
Rend.  Acad.  Sci.  Paris.     107 :  144. 
1889.     Sur  la  presence  des  composees  pectiques  dans  les  vegetaux.     Ibid.  109  :  579 
1890a.     Sur  la  substance  intercellulaire.     Ibid.  110 :  295. 

18906.     Sur  les  reactifs  colorants  des  substances  fondamentalea  de  la  membrane 
Ibid.  Ill:  120. 

1891.     Observations  sur  la  membrane  cellulosiq ue.     Ibid.  113;  lDii'.i. 
1893.     Sur  l'emploi  du  rouge  de  ruthenium  en  anatomie  vegetale.     Ibid.  116  .  653. 
McClendon,  J.  F.     1910a.     On  the  dynamics  of  cell-division.     I.     The  electric  chai 

on  colloids  in  living  cells  in  the  root  tips  of  plants.     Arch.  Entw  Bl:  80-90      nl 

figs.  2. 

19106.     On  the  dynamics  of  cell-division.     II.     Changes  in  perm < -ability  in  develop- 
.    ing  eggs  to  electrolytes.     Am.  Jour.  Physiol.  27 :  240-275. 

1913.     The  laws  of  surface  tension  and  their  applicability  "to  living  cells  and  cell 
division.     Arch.  Entw.  37:  233-247.     figs.  10. 
Meek,  C.  F.  U.     1913.     The  problem  of  mitosis.     Quar.  Jour.  Micr.  Sci.  68 :  567-592 
Meves,  Fr.     1896,  1898.     Zelltheilung.     Ergeb.  d.  An.it.  u.   Entw.  6:  285  390'  8~- 

430-542.     (Review.) 
von  Mohl,  H.   1853.     Ueber  die  Zusammensetzung  der  Zellmembran  aue  I  Ssern 
Bot.  Zeit.  11 :  753-762,  769-775. 
1858.     Die    Untersuchungen   des    Pnanzengewebes    mil     Bulfe   des    polaiisierten 
Lichtes.     Ibid.  16:  373-375.     1  pi. 
Moltsch,  H.     1888.     Zur  Kenntniss  der  Thyllen,  nebsl  Beobachtungen  QberWund- 
heilung  in  der  Pflanzen.     Sitzber.  k.  Akad.   Wiss.   Wien,   Math.-Naturw      CI 
I  97 :  264-299.     pis.  1,  2. 
Moreau,  F.     1913.    Les  Karyegamies  multiples  de  la  zygospore  '!<■  Rhizopua  ttitjri- 

cans.     Bull.  Soc.  Bot.  France  60:  121    123. 
Morgan,  T.  H.     1899.     The  action  of  sail  solutions  on  the  unfertilized  and  fertilized 
eggs  of  Arbacia,  and  of  other  animals.     Arch.   Entw.  8:    Ms  539.     p]<    -   m 
figs.  21. 


200  INTRODUCTION  TO  CYTOLOGY 

Mottier,  D.  M.     1900.     Nuclear  and  cell-division  in  Dictyota  dichotoma.     Ann.  Bot. 

14:  166-192.     pi.  11. 
von  Nageli,  C.     1858.     Die  Starkekorner.     Zurich. 

1863a.     Ueber  die  chemische  Zusammensetzung  der  Starkekorner  und  Zellmem- 

bran.     Ibid  1863.     (See  also  Nageli's  Bot.  Mitt.  1,  2.) 
18636.     Die  Anwendung  der  Polarisationsmikroscops  auf  die  Untersuchungen  der 

organischen  Elementartheile.     Beitr.  z.  Wiss.  Bot.  3 :  1-126.     pis.  1-7. 
1864.     Ueber   den  innern    Bau   der  vegetabilischen  Zellmembran.     Sitzber.  k.  b. 

Akad.     1864. 
Noll,  F.     1887.     Experimentaluntersuchungen  liber  d.  Wachsthum  d.  Zellmembran. 
Nothnagel,  M.     1916.     Reduction  divisions  in  the  pollen  mother-cells  of  Allium 

tricoccum.     Bot.  Gaz.  61:  453-476.     pis.  28-30.     fig.  1. 
Payen,  A.     1842.     Memoires  sur  les  developpements  des  vegetaux.     Paris. 
Pfeffer,  W.     1892.     Studien  der  Energetik. 
Prenant,  A.     1910.     Theories  et  interpretations  physiques  de  la  mitose.     Jour,  de 

l'Anat.  et  Phys.  46. 
Rabl,  C.     1889.     Ueber  Zellteilung.     Anat.  Anz.  4:  21-30.     figs.  2. 
Rhumbler,  L.     1896.     Versuch  einer  mechanischen  Erklarung  der  indirekten  Zell- 

und  Kerntheilung.     I.  Cytokinese.     Arch.  Entw.  3 :  527-623.     pi.  26.  figs.  39. 

1897.  Stemmen  die  Strahlen  der  Astrosphare  oder  ziehen  sie?     Ibid.  4:  659-730. 
pi.  28.     figs.  27. 

1898.  Die    Mechanik    der    Zelldurchschnurung    nach    Meves'  und  nach  meiner 
Auffassung.     Ibid.  7 :  535-554.     pi.  12.     figs.  5. 

1899.  Furchung  des  Ctenophoreneies  nach  Ziegler  und  deren   Mechanik:  usw. 
Ibid.  8:  187-238.     figs.  28. 

1903.     Mechanische  Erklarung  der  Aehlichkeit  zwischen  magnetischen  Kraftlinien- 
system  und  Zellteilungsfiguren.     Ibid.  16 :  476-535.     figs.  36. 
Robertson,  T.  B.     1909.     Note  on  the  chemical  mechanics  of  cell-division.     Arch. 
Entw.  27:29-34. 
1911.     Further  remarks  on  the  chemical  mechanics  of  cell-division.     Ibid.  32 :  308- 

313. 
1913.     Further  explanatory  remarks  concerning  the  chemical  mechanics  of  cell 
division.     Ibid.  35:  692-707.     figs.  3. 
Rytz,  W.     1907.     Beitrage  zur  Kenntniss  der  Gattung  Synchytrium.     Centr.  f .  Bakt. 

11  18 :  635-655,  799-825.     1  pi.    figs.  10. 
Sakamura,  T.     1920.     Experimented  Studien  liber  die  Zell-  und  Kernteilung  mit 
besonderer  Riicksicht  auf  Form,   Grosse  und  Zahl  der  Chromosomen.     Jour. 
Coll.  Sci.  Imp.  Tokyo  39:  pp.  221.     pis.  7. 
Sharp,  L.  W.     1911.     The  embryo  sac  of  Physostegia.     Bot.  Gaz.  62 :  218-225.     pis. 

6,  7. 
Spek,  J.     1918a.     Oberflaschenspannungsdifferenzen  als  eine  Ursache  der  Zellteilung. 
Arch.  Entw.  44:  1-113.     figs.  25. 
19186.     Die   amoboiden    Bewegungen   und   Stromungen   in   den   Eizellen   einiger 
Nematoden  wahrend  der  Vereinigung  der  Vorkerne.     Arch.  Entw.  44:  217-255. 
figs.  15. 
Strasburger,  E.     1875.     Ueber  Zellbildung  und  Zelltheilung.     Jena. 
1882.     Ueber  den  Bau  und  das  Wachstum  der  Zellhaute.     Jena. 
1884.     Die  Controversen  der  indirekten  Kernteilung.     Arch.  Mikr.  Anat.  23 :  246- 
304.     pis.  13,  14. 

1888.  Ueber  Kern-  und  Zellteilung  im  Pflanzenreich,  nebst  einem  Anhang  iiber 
Befruchtung.     Hist.  Beitr.  1.     pp.  258.     pis.  3. 

1889.  Ueber  das  Wachstum  vegetabilischer  Zellhaute.     Ibid.  2. 


THE  ACHROMATIC  FIGURE,  CYTOKINESIS,  AM)  CELL  WALL     201 

1892.  Schwarmsporen,  Gameten,  pflanzliche  Spermatozoiden  iind  das  Weeeo  der 

Befruchtung.     Ibid.  4:  49-158.     pi.  3. 

1897.  Ueber  Cytoplasmastrukturen,    Kern-  and  Zellteilung.     Jahrb.  Wise.   Bot. 

30 :  375-405.     figs.  2. 

1898.  Die  pfianzlichen  Zellhautc.     Ibid.  31 :  534-598.     pis.  15,  Hi. 
1907.     Apogamie  bei  Marsilia.     Flora  97:  123-191.     pis.  3-8. 

Swingle,  D.  B.     1903.     Formation  of  the  spores  in  the  sporangia  of  Rhizop  pi- 

cans  and  of  Phycomyces  niiens.     U.  S.  Dept.  Agric.  Bur.  Pit.  Indus.  Bull  37, 
Swingle,  W.  T.     1897.     Zur  Kenntniss  der  Kern-  und  Zelltheilung  bei  den  Sphace- 

lariaceen.     Jahrb.  Wiss.  Bot.  30:  296-350.     pis.  15,  1G. 
Taylor,  W.  R.     1920.     A  morphological  and  cytological  study  of  reproduction  in 

the  genus  Acer.  Contrib.  Bot.  Lab.  U.  of  Pa.  5:  pp.  30.     pis.  6-11. 
Timberlake,  H.  G.  1900.     The  development  and  function  of  the  cell  plate  in  higher 

plants.     Bot.  Gaz.  30 :  73-99.     154-170. 
1902.     Development  and  structure  of  the  swarm  spores  of  Hydrodictyon.     Trans. 

Wis.  Acad.  Sci.  13:  486-522.     pis.  29,  30. 
Tischler,  G.     Zellstudien  an  sterilen  Bastardpflanzen.     Arch.  Zellf.  1:  33-151. 
Treub,  M.     1878.     Quelques  recherches  sur  le  role  du  noyau  dans  la  division  d 

cellules  vegetales.     Amsterdam. 
Watase,  S.     1894.     Origin  of  the  centrosome.     Biol.  Lectures,  Woods  Hole. 
Wiesner,  J.     1864.     Untersuchungen  liber  das  Auftreten  von  Pectinkorper  in  dem 

Geweben  der  Runkelriibe.     Sitzber.  k.  Akad.  Wiss.     Wien,  Math.-Xaturw.     (1. 

1150:  442-453. 
1886.     Untersuchungen  iiber  die  Organization  der  vegetabilischen  Zellhaut.     Ibid. 

I  93 :  17-80.     figs.  5. 
Wigand,  A.     1856.     Ueber  die  feinste  Struktur  der  Zellenmembran.     Schriften  d. 

Ges.  z.  Beford.  d.  ges.  Xaturwiss.  zu  Wtirzburg. 
Williams,  C.  L.     1899.     The  origin  of  the  karyokinetic  spindle  in  Passiflora  car  ah  n. 

Proc.  Calif.  Acad.  Sci.  Ill  Bot.  1:  189-206.     pis.  33-40. 
Wtilson,  E.  B.     1899.     On  protoplasmic  structure  in  the  eggs  of  echinoderms  and 

some  other  animals.     Jour.  Morph.  15:  Suppl.     1-23. 

1900.  The  Cell  in  Development  and  Inheritance. 

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sis in  sea  urchin  eggs.     Arch.  Entw.  12:  529-596.     pis.  11-17.     figs.  12. 

Zacharias,  E.     1888.     Ueber  Kern  und  Zellteilung.     Bot,  Zeit.  46:  33-40,  51-62. 
•      pi.  2. 

1891.     Ueber  das  Wachstum  der  Zellhaut  bei  Wurzelhaaren.     Flora  74:  166   191. 
pis.  16,  17. 
Zimmermann,  A.     1887.     Die  Pflanzenzelle. 

1893.  Sammel-Referate.     6.     Beih.  Bot.  Centr.  3:  342-354 


CHAPTER  X 

OTHER  MODES  OF  NUCLEAR  DIVISION 

In  accordance  with  the  well  established  principle  which  states  that 
only  through  the  simpler  organisms  can  an  adequate  understanding  of 
those  higher  in  the  scale  of  complexity  be  approached,  search  has  been 
made  for  primitive  modes  of  nuclear  division  with  the  hope  that  light 
may  thereby  be  thrown  upon  the  origin  and  significance  of  the  elaborate 
karyokinetic  process  which  is  so  universally  found  in  the  cells  of  higher 
animals  and  plants.  It  is  to  be  acknowledged  that  such  a  phylogenetic 
explanation  of  mitosis  is  very  far  from  being  reached,  but  many  of  the 
observations  recorded  are  nevertheless  of  a  very  suggestive  nature.  To 
botanists  the  most  interesting  of  these  have  been  made  upon  the  Cyano- 
phyceae, which  have  long  been  a  subject  of  controversy  in  this  connection. 

Cyanophyceae. — For  many  years  the  nature  of  the  "central  body" 
of  the  cells  of  such  blue-green  algae  as  Oscillatoria  remained  very  obscure. 
Butschli  (1890),  Dangeard  (1892),  Scott  (1888),  and  others  believed  it 
to  be  a  nucleus  of  a  somewhat  primitive  type,  whereas  other  investigators, 
among  them  Zacharias  (1892)  and  Chodat  (1894),  denied  its  nuclear 
nature.  Zukal  (1892)  held  that  the  peripheral  portion  of  the  cell  repre- 
sents a  chromatophore,  the  central  body  consisting  of  cytoplasm  with  a 
number  of  minute  nuclei  imbedded  in  it. 

One  of  the  first  critical  accounts  based  partly  on  the  study  of  sections 
was  that  of  Fischer  in  1897.  Fischer  concluded  that  the  central  body, 
in  which  he  found  no  chromatin,  is  the  main  portion  of  the  cytoplasm, 
and  not  to  be  regarded  as  the  forerunner  of  the  nucleus  or  indeed  as  an 
independent  organ  at  all.  He  also  investigated  the  nature  of  the  periph- 
eral portion  of  the  protoplast.  By  treating-  the  plants  with  10  per 
cent  hydrofluoric  acid  he  dissolved  away  the  other  parts  of  the  cell,  leav- 
ing this  portion  intact;  and  as  the  result  of  comparative  studies  on  other 
plants  he  concluded,  in  harmony  with  Zukal,  that  it  is  a  single  large 
chromatophore. 

Since  Fischer's  work  the  most  important  contributions  are  those  of 

Hegler,   Kohl,   Olive,  Phillips,  Gardner,  and  Miss  Acton.     Contrary  to 

the  view  of  Fischer,  all  of  these  cytologists  interpret  the  central  body 

as  a  nucleus,  and  the  first  three  regard  its  division  as  essentially  mitotic. 

The  opinions  of  these  workers  with  respect  to  the  organization  of  the  cell 

of  the  Cyanophyceae  and  the  behavior  of  its  nucleus  are  summarized 

below. 

202 


OTHER  MODES  OF  NUCLEAR  DIVISION 


203 


According  to  Hoglcr  (1901)  the  nucleus  contains  granules  of  chromatin 
but  no  nucleolus  or  nuclear  membrane,  division  occurring  by  a  simple 
form   of  mitosis.     The   coloring   mailer   exists   in   the   form    of   mini, 
granules  or  cycmoplasts.     Two  other  kinds  of  bodies  are  also  present: 
albuminous  slime  globules  and  albuminous  crystals  [cyanophycin  granul 
representing  reserve  food. 

KohFs  (1904)  description  of  the  cell  of  Tolypothrix  I  Fig.  70    Is  one  of 
the  most  detailed  which  has  been  given  in  this  group  of  research  Kohl 

shows  that  the  nucleus  of  this  form  has  extensions  reaching  outward 


a \V.  *'.'■'  ■ ' ' — -* 

ft 


\m\ 

0©? 

mill 

OOC 

Fig.  70. — Structure  and  division  of  the  cell  of  Tolypothrix  lanata. 
A,  cell  in  the  vegetative  state:  c,  cytoplasm;  n,  nucleus;/,  fat  droplets;  />.  phycocyanin 
and  chlorophyll  granules;  s,  slime  globules;  g,  granules  of  cyanophycin.      />'.  foul 
cell-division  in   Tolypothrix,   showing  transverse  division  of  chromosomi  diagram 

showing  6  stages  of  cell-division.      (After  Kohl,  1903.) 


toward  the  cell  wall,  and  that  they  are  withdrawn  at  the  time  of  Qiiclear 
division.  The  nucleus,  which  contains  chromatin,  also  includes  a  num- 
ber of  large  Zentralkorner,  or  slime  globules,  while  in  the  cytoplasm  are 
fat  droplets,  cyanophycin  granules  of  reserve  albumen,  and  granules  of 
chlorophyll  and  phycocyanin.  The  nucleus,  which  is  very  rarely  in  the 
resting  state,  divides  as  follows:  the  chromatic  material  forms  a  spireme 
which  segments  into  a  definite  number  of  chromosomes;  these  lie  in  the 
direction  of  the  long  axis  of  the  cell  and  break  transversely  as  the  separat- 
ing wall  grows  inward  from  the  periphery.  Their  halves  are  thus  in- 
cluded in  the  two  daughter  cells,  where  they  form  daughter  nuclei  without 
membranes. 


204 


INTRODUCTION  TO  CYTOLOGY 


Fig.  71. — Nuclear  division 
in  Oscillatoria  Froelichia.  1,  2, 
3,  4,  four  successive  stages. 
{After  Olive,  1904.)  j 


Olive  (1904) x  finds  that  the  nucleus  of  Oscillatoria  (Fig.  71)  consists 
of  a  fibrous  achromatic  framework  with  a  number  of  very  small  chroma- 
tin granules,  and  is  nearly  always  in  some  stage  of  division.  A  spireme 
is  formed  carrying  16  chromatin  granules  (8  in  Glceocapsa  and  32  in  one 
species  of  Oscillatoria) ,  each  representing  a  chromosome.  The  spireme 
and  its  chromatin  granules  are  split  longitudinally,  and  the  daughter 
spiremes  with  the  daughter  granules  separate,  a  distinct  central  spindle 
extending  between  them.     The  dividing  wall  is  formed  as  a  centripetally 

growing  partition.  In  Glceocapsa  the  cell  di- 
vides by  simple  constriction.  The  vegetative 
nuclei  of  Oscillatoria  very  rarely  approach 
the  resting  condition,  but  in  spores  and 
heterocysts  they  soon  pass  into  this  state,  a 
nuclear  membrane  and  vacuole  being  de- 
veloped. In  the  heterocyst  the  protoplast 
disorganizes.  Olive  regards  the  central  body 
of  the  Cyanophycese  as  not  essentially  different 
from  the  nucleus  of  the  higher  plants,  although 
it  is  relatively  primitive  in  several  features. 
In  the  cytoplasm  he  finds  both  cyanophycin 
granules  and  slime  globules,  but  no  cyano- 
plasts,  the  coloring  matters  being  diffused  in  the  peripheral  portion 
of  the  protoplast. 

Fischer  (1905),  in  reply  to  the  claims  of  Kohl  and  Olive,  reasserted 
his  view  that  the  central  body  is  not  a  nucleus,  but  rather  an  accumulation 
of  carbohydrate  materials.  The  glycogen  formed  as  a  result  of  assimila- 
tory  activity  gathers  in  the  central  body  where  it  is  transformed  into 
another  carbohydrate,  anabcenin,  which  assumes  the  form  of  sausage- 
shaped  structures.  At  the  time  of  cell-division  these  masses  of  reserve 
material  are  distributed  by  a  process  of  "pseudomitosis"  to  the  daughter 
cells.  Fischer  therefore  regards  the  mitotic  figures  observed  by  others 
as  significant  in  connection  with  nutrition  rather  than  with  the  functions 
usually  attributed  to  nuclei. 

Gardner  (1906),  investigating  a  number  of  species,  found  nuclei  of 
three  kinds,  which  he  called  the  diffuse  type,  the  net  karyosome  type, 
and  the  primitive  mitosis  type  respectively.  The  " diffuse  type"  of 
nucleus,  which  has  no  very  definite  delimitation  from  the  peripheral 
portion  of  the  protoplast,  contains  an  indefinite  number  of  chromatin 
masses.  As  the  cell  divides  this  central  aggregation  of  chromatic  material 
divides  into  approximately  equal  portions.  In  the  "net  karyosome" 
type,  found  in  Dermocarpa,  the  distinction  between  the  nucleus  and  the 
surrounding  cytoplasm  is  much  clearer.     The  nucleus  has  an  achromatic 

1  A  very  convenient  tabulation  of  the  results  of  researches  on  cell  structure  in  the 
Cyanophycese  up  to  1904  is  given  by  Olive. 


OTHER  MODES  OF  NUCLEAR  DIVISION 


2<>:> 


network  with  chromatin  granules  at  its  nodes,  and  constricts  simultai 
ously  into  a  large  number  of  daughter  nuclei  which  pass  fco  the  conidia. 
In  Synochocystis  aquatilis  occurs  the  "primitive  mitosis fl  type:  here 
Gardner  found  the  only  case  of  anything  approaching  mitotic  behavior. 
A  spireme  develops  and  segments  into  three  pieces  which  arrange  them- 
selves parallel  to  the  long  axis  of  the  cell  and  divide  transversely;  the 
daughter  pieces  then  separate  and  a  centripetally  growing  cell  wall 
completes  the  division  of  the  cell.  Gardner  thus  finds  in  the  Cyano- 
phyceae  "a  series  of  nuclear  structures,  beginning  with  a  very  simple 


Fig.  72. — The  nuclei  of  various  members  of  the  Chroococcacese. 

A,  cell  of  Chroococcus  turgidus  with  scattered  metachromatin  granules  (to)  and  plasma- 
tic microsomes  (p) ;  division  beginning.  X  2500.  B,  cell  of  Gloeocapsa  with  chromatic 
granules.  C,  M erismopedia  elegans,  showing  two  stages  of  nuclear  division.  X  1500. 
D.  Chroococcus  macrococcus:  n,  nucleus;  m,  metachromatin;  v,  vacuole.  X  2-"><>i>.  /.. 
dividing  nucleus  of  Chroococcus  macrococcus.      X  2500.     (After  Acton,  1914. 


form  of  nucleus  scarcely  differentiated  from  the  surrounding  cytoplasm 
and  dividing  by  simple  direct  division"  and  passing  "by  very  gradual 
steps  to  a  highly  differentiated  form  of  nucleus  which  in  dividing  shows 
a  primitive  type  of  mitosis,  and  in  structure  approximates  the  nucleus  of 
the  Chlorophyceae  and  the  higher  plants." 

In  a  more  recent  investigation  of  the  Chroococcaceae  Miss  Act  on 
(1914)  finds  that  the  nucleus  is  in  general  much  simpler  than  that  of  the 
higher  plants.  Like  Gardner,  however,  she  points  out  a  series  beginning 
with  a  form  in  which  definite  organization  is  almost  entirely  lacking  and 
ending  with  one  in  which  the  structure  of  the  higher  plant  nucleus  is 
closely  approached  (Fig.  72).     In  Chroococcus  turgidus  the  protoplast  is 


206  INTRODUCTION  TO  CYTOLOGY 

made  up  of  a  ground  substance  with  a  reticulum  bearing  bodies  of  two 
sorts:  granules  of  metachromatin  closely  similar  to  chromatin  in  reaction, 
and  cyanophycin  granules,  or  plasmatic  microsomes.  Although  there  is 
no  definitely  delimited  central  region  in  the  cell  the  metachromatin  is 
found  mostly  at  the  center  and  the  cyanophycin  mostly  nearer  the  pe- 
riphery. When  the  metachromatin  granules  become  numerous  division 
sets  in,  a  centripetally  growing  wall  cleaving  the  protoplast  into  two 
daughter  cells.  In  Gloeocapsa  the  central  region  is  somewhat  more  definite 
and  may  often  show  a  spireme-like  appearance  such  as  Olive  describes; 
but  this  may  possibly  be  an  artifact.  In  Merismopedia  elegans  there  is  a 
definitely  delimited  nucleus,  not  like  that  of  the  higher  plants  but  merely 
an  accumulation  of  chromatin  or  chromatin-like  material  which  divides 
just  before  the  cell  constricts  into  two  portions.  In  Chroococcus  macro- 
coccus,  finally,  the  nucleus  and  cytoplasm  are  sharply  distinct,  the  former 
having  a  reticulum  with  chromatin  granules  at  its  nodes  and  dividing 
by  a  sort  of  constriction  at  the  time  of  cell-division. 

As  a  result  of  these  observations  Miss  Acton  advances  a  theory  of  the 
evolution  of  nucleus  and  cytoplasm,  which  is  briefly  as  follows.  The 
excess  food  elaborated  by  the  protoplast  with  its  pigments  was  first 
stored  as  plasmatic  microsomes  composed  of  a  carbohydrate,  cyanophy- 
cin. As  the  reserve  material  became  more  complex  in  nature  the  nucleo- 
protein  metachromatin  was  elaborated;  this  became  aggregated  at  the 
center  of  the  cell,  insuring  its  equal  distribution  in  cell-division,  as  in 
Merismopedia.  There  thus  arose  in  the  cell  a  physiological  and  mor- 
phological differentiation,  the  nucleo-protein  with  its  portion  of  the 
supporting  reticulum  becoming  a  stable  nucleus,  as  in  Chroococcus 
macrococcus,  and  the  ground  substance  remaining  as  the  cytoplasm. 

Summary. — In  the  Cyanophycese,  therefore,  although  these  forms  in 
all  probability  had  nothing  directly  to  do  with  the  evolution  of  the  higher 
plants,  we  see  a  series  of  stages  such  as  may  well  have  occurred  in  the 
evolution  of  the  nucleus  and  its  complicated  mitotic  division.  In  the 
simplest  forms  the  material  concerned  with  those  cell  activities  which 
in  higher  organisms  are  associated  with  the  nucleus,  is  scattered  through- 
out the  cell  without  the  morphological  distinctness  characteristic  of  an 
organ  in  the  strict  sense.  It  is  passively  distributed  to  the  daughter 
cells  when  the  cleavage  wall  is  formed  at  the  time  of  cell-division.  In 
other  cases  this  material  reacts  more  strongly  like  true  chromatin  and 
may  form  a  more  or  less  definite  aggregation  separating  into  two  masses 
as  the  cell  divides.  This  metachromatin,  which  is  a  nucleic  acid  com- 
pound, has  also  been  observed  in  other  algaB,  in  Protozoa,  and  in  fungi, 
including  the  yeasts.  It  appears  to  represent  a  reserve  material,  though 
it  may  also  have  other  functions.  Finally,  definite  and  well  organized 
nuclei  are  present  in  certain  of  the  forms  described  in  the  foregoing 
pages,  and  although  these  nuclei  may  lack  some  of  the  features  exhibited 


OTHER  MODES  OF  NUCLEAR  DIVISION 


207 


by  the  nuclei  of  higher  organisms,  they  show  in  the  division  and  distribu- 
tion of  their  chromatic  elements  many  of  the  characteristics  of  true 
mitosis.     In  the  evolution  of  the  nucleus  through  such  a  series  of  stag 
we  have  an  illustration  of  "the  conception  of  cell  structure  which  im- 


*  "  V 


B 


a 

f>  '*%%&'       m 

,Vff; 

.■wflftrMp                   Tit} 

P*.t><    VSjil  ■■m<r.-  $               ■ZxmB 

/ 


%^«fc./' 


Fig.  73. — Nuclear  division  in  Protozoa. 
.4,  one  form  of  mitosis  in  Amoeba  diplomitotica.      (From  Minchin,  after  Aro'ja,*.       />. 
Nuclear   division   in    Coccidium    schubergi.      X  2250.     {From   Minchin,  a         v  haudii 
C,  mitotic  division  of  micronucleus  of  Paramecium   (horizontal  figure  on  Bmaller  scale 

than  others.)      {From  Minchin,  after  Hertwig.) 

plies  differentiated  regions  of  a  colloidal  system  in  which  special  processes 
have  become  localized  and  tend  to  remain  fixed"    Harper  L919). 

Protozoa. — Such  an  apparent  derivation  of  mitosis  from  a  simpler, 

more   indefinite   division   of   a   less    sharply   delimited    mass   of   special 


208  INTRODUCTION  TO  CYTOLOGY 

chromatic  substance  is  seen  also  in  the  protozoa.  Here  the  chromatic 
material  in  a  number  of  species  is  held  together  in  loose  granular  aggrega- 
tions, and  the  achromatic  figure  is  seen  in  curious,  relatively  simple 
manifestations.  In  many  cases,  however,  even  among  the  Rhizopoda, 
there  occurs  a  very  advanced  type  of  mitosis,  with  spindle,  centrosomes, 
asters,  and  a  definite  number  of  chromosomes. 

One  of  the  simplest  types  of  nuclear  division  found  among  the  pro- 
tozoa is  known  as  "chromidial  fragmentation."  Here  the  nucleus  is 
resolved  into  a  large  number  of  chromatin  granules,  or  chromidia,  which 
reassemble  in  two  or  more  groups  and  form  new  nuclei.  In  nuclei  of  the 
"  vesicular  type,"  found  commonly  among  the  Microsporidia,  the  chroma- 
tin is  concentrated  in  a  single  large  body,  or  karyosome,  which  becomes 
dumbbell-shaped  and  divides,  the  rest  of  the  nucleus  then  dividing  also. 
In  other  forms,  such  as  Coccidium  (Fig.  73,  B),  the  nucleus  contains  in 
addition  a  second  kind  of  chromatin  which  is  approximately  halved  in 
nuclear  division.  It  is  but  a  short  step  from  such  non-mitotic  division 
as  this  to  the  simplest  types  of  mitosis  (" promitosis")  seen  in  many 
protozoa.  Within  the  group  are  found  all  gradations  in  complexity 
from  such  primitive  modes  of  division  up  to  the  advanced  types  showing 
a  complete  achromatic  figure  with  chromosomes  which  are  regular  in 
form,  number,  arrangement,  and  division,  just  as  in  the  higher  animals.1 

Both  Metcalf  and  Kofoid  (1915)  have  emphasized  the  fundamental 
similarity  of  protozoan  and  metazoan  nuclei.  The  process  of  mitosis 
has  the  same  succession  of  phases  in  the  two  cases,  though  many  minor 
variations  occur.  In  some  representatives  of  all  the  main  groups  of 
protozoa  are  found  elongated  chromosomes  which  Metcalf  regards  as 
linear  aggregates  of  chromatin  granules,  and  which  split  longitudinally, 
giving  exact  equivalence  to  the  daughter-nuclei.  Although  the  cell 
mechanism  of  Mendelian  inheritance  is  thus  held  to  be  present  in  members 
of  each  great  protozoan  group  and  to  operate  as  in  the  metazoa  at  the 
sexual  stages,  Metcalf  believes  this  mechanism  is  not  kept  intact  through 
the  vegetative  phases  as  it  is  in  the  higher  groups. 

Other  Cases  in  Plants. — With  regard  to  the  myxomycetes,  the 
researches  of  Strasburger  (1884),  Harper  (1900),  Jahn  (1904,  1911), 
Olive  (1907),  and  Winge  (1912)  have  shown  that  nuclear  division  is 
essentially  mitotic,  and  that  in  some  cases  the  chromosomes  are  not 
only  definite  in  number  but  undergo  a  reduction  prior  to  spore  formation. 
As  an  example  of  an  exceptional  condition  may  be  taken  Sorodiscus 
(Fig.  74,  A),  in  which  Winge  describes  two  sorts  of  chromatin:  vegetative 
tropho chromatin  and  generative  idiochromatin,  the  two  forming  a  single 
mass  at  the  center  of  the  nucleus.  As  nuclear  division  begins  this  mass 
takes  the  form  of  three  or  four  bodies  very  similar  to  chromosomes. 

1  For  a  description  of  mitotic  phenomena  in  protozoa  see  Minchin  (1912,  Chapter 
VII). 


OTHER  MODES  OF  XCCLEAK  DIVISION 


209 


The  two  kinds  of  chromatin  now  separate,  the  "trophochromatiE  placing 
itself  in  the  center  and  the  generative  or  idiochromatin  lying  like  a  thin 
equatorial  plate  around  it."  As  the  nucleus  elongates  the  tropho- 
chromatin  body  becomes  dumbbell-shaped  and  breaks  into  two,  while 
the  idiochromatin  plate  splitsvinto  daughter  plates  which  apparently 
move  to  the  poles  and  cooperateVith  the  trophochromatin  in  the  forma- 
tion of  the  daughter  nuclei. 


■ 

Fig.   74. 

A,  two  stages  of  mitosis  in  Sorodiscus.  (After  Winge,  1912.)  B,  anaphase  of  nuclear 
division  in  Euglena.  Chromosomes  grouped  about  dividing  "  nucleolo-centrosome." 
(After  Keuten,  1895.)  C,  chromosomes  developing  from  nucleolus  in  Spirogyra.  X  1335. 
(After  Berghs,  1906.)      D,  Mitosis  in  Spirogyra  crassa.     (After  Merriman,  1913.) 

A  process  with  much  the  same  appearance  at  certain  stages  is  seen  in 
the  flagellate,  Euglena  (Keuten  1895)  (Fig.  74,  B).  Here  the  chromo- 
somes group  themselves  about  the  large  nucleolus  which  soon  takes  the 
form  of  a  dumbbell-shaped  " central  spindle,:  or  " centrodesmose." 
The  nucleolus  completes  its  division,  the  chromosomes  meanwhile 
separating  into  two  groups  which  pass  to  the  poles  and  reorganize  the 
daughter  nuclei.  In  certain  other  flagellates  Kofoid  (1915)  reports  a 
split  spireme  and  a  definite  number  of  chromosomes  which  differ  markedly 
in  size  and  shape. 

In  Cladophora  (Carter  1919)  nearly  all  the  chromatin  is  contained  in 
one  or  more  large  chromatin  nucleoli,  or  karyosomes.     After  the  numer- 
ous chromosomes  have  arrived  at  the  two  poles  al  the  close  of  the  ana- 
phase the  spindle  connecting  the  two  groups  constricts  and  complel 
the  division  of  the  nucleus. 

Another  unusual  condition  is  found  in  Spirogyra   (Fig.   74,  C,   D 
In  this  form  nearly  all  of  the  chromatic  material  is  Lodged  in  the  larf 
nucleolus,  the  nuclear  reticulum  being  very  delicate  and  almost  invisible 
in  many  preparations.     According  to  Berghs   (1906),   Karstm   Minis 
and  Trondle  (1912)  all  the  chromosomes  which  appear  in  the  propha 
and  split  as  usual  are  derived  from  this  nucleolus,  most  of  its  material 

14 


210  INTRODUCTION  TO  CYTOLOGY 

being  used  in  their  formation.  In  the  opinion  of  Miss  Merriman(1913) 
the  chromatic  bodies  observed  by  the  above  workers  are  not  true  chromo- 
somes, but  are  rather  more  indefinite  chromatic  aggregations  which  are 
variable  in  number  and  appearance,  and  which  are  irregularly  pulled 
apart  as  mitosis  proceeds.  She  finds  here  "no  evidence  throughout  the 
karyokinesis  of  an  equational  division  of  autonomous  bodies." 

In  Zygnema  both  Escoyez  (1907)  and  van  Wisselingh  (1914)  find  that 
the  reticulum,  and  not  the  nucleolus,  gives  rise  to  all  the  chromosomes. 
Although  the  nucleolus  furnishes  no  morphological  element,  chromatic 
material  may  flow  from  it  to  the  chromosomes  as  they  develop  from  the 
reticulum.  Much  the  same  condition  is  found  in  Marsilia  (Strasburger 
1907;  Berghs  1909).  Strasburger  points  out  that  in  the  somatic  nuclei 
(in  the  cells  of  the  root  and  the  young  prothallium)  most  of  the  chromatic 
substance  is  held  in  the  nucleolus  during  the  resting  stages  (Fig.  17,  E), 
and  that  the  material  of  the  reticular  framework,  which  is  very  delicate, 
is  to  be  regarded  as  the  substance  of  importance  in  heredity.  Berghs 
shows  that  the  nucleolus  consists  of  an  achromatic  substratum  which 
appears  independently  of  the  reticulum  in  the  telophase  and  soon  becomes 
impregnated  with  chromatic  material  transferred  to  it  from  the  chromo- 
somes. In  the  next  prophase  the  chromatic  material  flows  back  to  the 
delicate  reticulum,  from  which  the  chromosomes  gradually  develop.  As 
the  chromosomes  increase  in  distinctness  the  nucleolus  becomes  paler, 
and  when  the  nuclear  membrane  breaks  down  the  nucleolus  dissolves  in 
the  protoplasmic  liquid.  It  is  therefore  clear  that  in  Marsilia  the  nu- 
cleolus is  not  a  mere  aggregation  of  the  chromosomes  of  the  telophase,  as 
might  at  first  be  supposed.  The  chromosomes  arise  from  the  reticulum 
as  usual,  and  not  from  the  nucleolus  as  reported  for  Spirogyra.  In  these 
observations  we  have  additional  evidence  favoring  the  view  of  Haecker, 
Boveri,  Marechal,  and  others  (see  Chapter  VIII)  that  it  is  the  achromatic 
substratum  of  the  chromosome,  and  not  the  chromatic  substance  which 
it  carries,  that  should  be  regarded  as  the  persistent  structural  unity 
representing  the  basis  of  inheritance. 

Amitosis. — In  amitotic  or  direct  nuclear  division  the  nucleus  simply 
constricts  and  separates  into  two  portions  while  in  the  " resting"  condi- 
tion, no  condensed  chromosomes,  centrosomes,  spindle,  or  asters  being 
formed.  As  a  general  rule  such  a  division  of  the  nucleus  is  not  followed 
by  a  division  of  the  cell ;  cells  with  two  or  more  nuclei  therefore  commonly 
result.  As  examples  may  be  cited  the  tapetal  cells  in  the  anthers  of 
angiosperms,  the  internodal  cells  of  Char  a  (Fig.  75)  (Johow  1881),  and 
certain  glandular  cells  of  animals.  The  presence  of  more  than  one 
nucleus  cannot  by  itself  be  regarded  as  evidence  that  amitosis  has 
occurred,  however.  Amitosis  appears  to  be  of  rather  frequent  occurrence 
among  the  lower  organisms,  some  of  which  show  other  methods  of  divi- 
sion also.     For  example,  amitosis  occurs  regularly  in  budding  yeasts, 


OTHER  MODES  OF  NUCLEAR  DIVISION 


211 


^ 


though  the  divisions  giving  rise  to  the  ascospore  nuclei  have  been  shown 
to  be  mitotic  in  certain  cases.  (See  Guilliermond  L920.)  Amitosis 
was  once  believed  to  be  the  normal  mode  of  nuclear  division,  mitosis 
being  looked  upon  as  very  exceptional.  The  true  condition,  so  far  as 
higher  organisms  are  concerned,  has  turned  out  to  be  quite  the  revei 
it  is  evident  that  amitosis  occurs  frequently  in  certain  kinds  of  cells,  but 
the  mitotic  method  of  division  has  been  found  to  be 
almost  universal. 

What  the  physiological  significance  of  amitosis 
may  be  is  not  well  known.  It  was  once  suggested 
(Chun  1890)  that  it  aids  the  processes  of  metabolism 
by  increasing  the  nuclear  surface  in  the  cell,  since 
it  is  of  such  frequent  occurrence  in  cells  with  a  dis- 
tinctively nutritive  function.  This  view  has  recently 
been  restated  by  Nakahara  (1917)  as  a  result  of  his 
work  on  the  larva  of  Pieris.1  The  most  generally  held 
opinion  regarding  amitosis  in  the  higher  organisms 
was  for  many  years  that  expressed  by  Flemming 
(1891),  namely,  that  it  represents  a  degeneration 
phenomenon  or  aberration  of  some  kind,  which  would 
explain  why  it  is  so  often  found  in  degenerating  and 
pathological  tissues.  In  the  words  of  vom  Rath 
(1891),  "when  once  a  cell  has  undergone  amitotic 
division  it  has  received  its  death-warrant;  it  may  indeed  continue  to 
divide  for  a  time  by  amitosis,  but  inevitably  perishes  in  the  end.'' 

That  the  view  of  vom  Rath  must  be  modified  has  been  indicated  1)V 
the  results  of  a  number  of  investigations.  For  instance.  Pfeffer  (J899 
and  Nathansohn  (1900)  found  that  if  Spirogyra  filaments  are  placed  in  a 
J£  to  1  per  cent  solution  of  ether  the  nuclei  divide  by  amitosis  only,  and 
that  when  the  filaments  are  returned  to  pure  water  the  mitotic  method 
of  division  is  resumed,  with  no  evidence  of  degeneration.  Haecker,  how- 
ever, working  on  the  eggs  of  Cyclops,  came  to  view  such  artificially  in- 
duced behavior  not  as  true  amitosis  but  rather  as  a  much  modified 
mitotic  division,  which  he  termed  "pseudoamitosis."  Other  cytologiste 
observed  nuclear  divisions  that  seemed  intermediate  in  character  hit  ween 
mitosis  and  amitosis  (Dixon  in  the  endosperm  of  Fritillaria,  L895;  Sargant 
in  the  embryo  sac  of  Lilium,  1896;  R.  Hertwig  in  Actinosphaerium,  L898; 
Buscalioni  in  the  endosperm  of  Corydnlis,  1898;  and  \\  asielewski  in  tin- 
roots  of  Vicia  faba,  1902,  1903).  Hertwig  accordingly  concluded  that 
mitosis  and  amitosis  are  separated  by  no  sharp  boundary  line,  hut  are 
connected  by  an  unbroken  series  of  transition  stages. 

1  In  a  second  paper  (1918)  Nakahara  gives  a  convenienl  review  of  tin'  literature 

of  the  subject. 


Fig.  7.">.  Amitosis 
in  internodaJ  cell  of 
Cham.      X  413. 


212 


INTRODUCTION  TO  CYTOLOGY 


As  a  result  of  his  recent  researches  on  chloralized  cells  (Fig.  76) 
Sakamura  (1920)  interprets  all  such  unusual  types  of  nuclear  division  as 
those  described  by  Hertwig  and  Wasielewski  as  the  effect  of  disturbed 
mitotic  division,  but  denies  the  claim  of  those  authors  that  such  types 
of  division  represent  actual  transition  stages  between  amitosis  and 
mitosis.  True  amitosis  he  regards  as  a  fundamentally  different  process, 
and  as  essentially  a  degeneration  phenomenon. 


D 


F 


Fig.  76. — Abnormal  mitosis    in  chloralized  root  cells  of   Vicia. 

A,  chromosomes  distributed  irregularly  in  cell.  B,  scattered  chromosomes  beginning  to 
assume  nuclear  form.  C,  nucleus  reconstructed  by  scattered  chromosomes.  D,  scattered 
chromosomes  reconstructing  3  separate  nuclei.  E,  chromosomes  reconstructing  2  nuclei 
connected  by  bridge.  F,  amitosis-like  appearance  resulting  from  condition  shown  in  E. 
{After  Sakamura,  1920.) 

On  the  contrary,  Des  Cilleuls  (1914)  reports  that  in  the  rabbit 
periods  of  amitosis  and  mitosis  succeed  each  other  regularly  in  the  same 
cell  lineage  without  affecting  the  vitality  of  the  cells.  In  his  opinion, 
therefore,  amitosis  does  not  necessarily  place  the  stigma  of  senescence 
upon  the  cell.  A  similar  conclusion  is  reached  by  Arber  (1914),  who  finds 
amitosis  supplementing  mitosis  in  the  early  growth  stages  of  the  leaves 
and  adventitious  roots  of  Stratiotes  aloides;  and  by  McLean  (1914)-,  who 
asserts  that  it  is  the  sole  method  of  nuclear  division  in  the  cortical 
parenchyma  of  several  aquatic  angiosperms.  Saguchi  (1917)  likewise 
states  that  the  nuclei  in  the  ciliated  cells  of  vertebrates  divide  by  amitosis 

only. 

Amitosis  and  Heredity. — One  of  the  most  important  theoretical  ques- 
tions raised  by  the  phenomenon  of  amitosis  is  that  of  the  effect  which 
the  process  may  have  upon  the  hereditary  mechanism  of  the  cell.  Ac- 
cording to  the  chromosome  theory  of  heredity  and  development  in  its 
usual  form  it  has  been  thought  that,  although  amitosis  may  occur  in 
connection  with  an  altered  metabolism  in  cells  not  to  undergo  further 
differentiation,  mitosis  must  occur  exclusively  in  the  germ  cell  lineage, 
in  order  that  the  chromosomes  and  the  hereditary  elements  they  con- 


OTHER  MODES  OF  NUCLEAR  DIVISION  213 

tain  shall  be  properly  distributed  to  the  reproductive  cells;  and  also  in 
developing  tissues  and  organs,  so  that  differentiation  may  proceed  nor- 
mally. On  the  other  hand,  several  workers  (Meves;  Flemming  in  hie 
later  papers)  admit  that  amitosis  may  not  affect  any  hereditary  powers 
which  the  nuclei  concerned  may  possess.  Child  (1907,  1911),  who  re- 
ports amitosis  in  both  the  somatic  and  germ  colls  of  certain  animals, 
where  it  appears  to  play  an  important  role  in  the  developmental  cycle, 
strongly  urges  that  such  facts  render  the  hypothesis  of  chromosome  in- 
dividuality highly  improbable,  and  that  our  conceptions  of  the  role  of 
the  cell  organs  in  heredity  must  be  greatly  altered. 

The  hopelessly  unsettled  state  of  opinion  on  this  question  may  be 
illustrated  by  the  list  of  authors  and  their  views  cited  by  Conklin  (1917) . 
That  amitosis  frequently  occurs  in  the  process  of  normal  cell  differ- 
entiation, and  therefore  constitutes  evidence  against  the  chromosome 
theory,  has  been  held  by  Nathansohn  (1900),  Wasielewski  (1902,  1903), 
Gurwitsch  (1905),  Hargitt  (1904,  1911),  Child  (1907,  1911),  Patterson 
(1908),  Glaser  (1908),  Jordan  (1908),  Jorgensen  (1908),  Maximow  (1908), 
Moroff  (1909),  Knoche  (1910),  Nowikoff  (1910),  and  Foot  and  Strobell 
(1911).     Several  of  these  investigators,  together  with  R.  Hertwig  (1898), 
Lang   (1901),   Calkins   (1901),   Herbst   (1909),    Godlewski   (1909),   and 
Konopacki  (1911),  see  no  principal  distinction  between  amitosis  and 
mitosis,  believing  that  both  may  occur  without  interfering  with  normal 
differentiation. 

Haecker  (1900),  Nemec  (1903),  and  Schiller  (1909)  dissented  from  the 
above  view,  which  was  also  strongly  contested  by  Boveri  (1907;  and 
Strasburger  (1908).  Richards  (1909,  1911)  and  Harman  (1913)  failed  to 
confirm  the  results  of  Child  on  amitosis  in  cestodes,  but  Child  (1911) 
reasserted  his  view,  which  was  supported  by  Young  (1913).  Schurhoff 
(1919),  working  on  Podocarpus,  emphatically  states  that  a  nucleus  which 
has  once  undergone  true  amitosis  is  incapable  of  dividing  mitotieally. 
Sakamura  (1920)  is  of  the  same  opinion. 

In  a  careful  study  of  maturation  and  cleavage  in  Crcpidula  pla> 
Conklin  (1917)  finds  that  the  nuclei  divide  only  by  mitosi-.  There  are 
many  apparent  cases  of  amitosis,  but  upon  careful  examination  they  all 
prove  to  be  only  various  modifications  of  the  regular  mitotic  process. 
Such  modifications  are  these:  the  scattering  of  the  chromosome  and  their 
failure  to  unite  into  a  single  nucleus;  mitosis  without  cytokinesis,  Lri\ ing 
cells  with  two  or  more  nuclei;  the  failure  of  certain  daughter  chromosomi 
to  pull  apart,  leaving  a  chromatic  bridge  between  the  daughter  nuclei, 
the  persistence  of  the  nuclear  membrane,  with  a  division  of  the  chromo- 
somes by  mitosis  and  of  the  nuclear  vesicle  by  constriction.  Conklin 
concludes  as  a  result  of  his  many  observations  and  an  examination  of 
the  evidence  offered  by  others,  that  there  is  not  known  a  single  <. inclusive 
case  of  true  amitosis  in  a  normally  differentiating  cell,  and  that  all  attacks 


214  INTRODUCTION  TO  CYTOLOGY 

upon  the  chromosome  theory  on  the  ground  of  amitosis  have  signally 
failed.  The  results  obtained  by  Sakamura  (1920)  in  his  study  of  modi- 
fied mitosis  in  chloral ized  plant  cells  are  strikingly  similar  to  those  of 
Conklin,  and  his  conclusions  regarding  the  chromosome  theory  are  es- 
sentially the  same. 

From  the  foregoing  it  is  evident  that  the  problem  of  the  effect  of 
amitosis  upon  the  differentiation  of  the  tissues  in  which  it  occurs  and 
upon  the  hereditary  powers  of  the  nucleus  is  by  no  means  easy  of  solution, 
and  that  much  care  must  be  used  in  interpreting  supposed  amitotic  phe- 
nomena in  fixed  preparations.  The  work  of  Conklin  and  Sakamura  has 
shown  clearly  that  many  of  the  phenomena  reported  as  amitosis  are  in 
reality  aberrations  of  the  mitotic  process,  and  that  the  opinions  of  many 
writers  are  undoubtedly  due  to  a  failure  to  recognize  this  fact.  Should  it 
be  proved,  however,  that  true  amitosis  may  occur  in  the  lineage  of 
normally  functioning  germ  cells  a  serious  obstacle  would  be  placed  in  the 
way  of  the  chromosome  theory  of  inheritance  in  its  current  form,  for  this 
theory  requires  that,  no  matter  what  happens  in  cells  not  in  the  direct  line 
of  the  germ  cells,  nuclear  division  in  this  line  must  be  exclusively  mitotic 
in  order  that  the  hereditary  mechanism  in  the  nucleus  shall  be  preserved. 
This  mechanism,  as  we  shall  see  in  later  chapters,  is  supposed  to  be  of 
such  a  nature  that  amitosis  would  seriously  derange  its  organization. 
In  each  daughter  nucleus  of  an  amitotic  division  some  of  the  elements 
necessary  for  normal  functional  activity  would  presumably  be  lacking, 
owing  to  the  simple  mass  division  of  the  chromatin.  With  reference  to 
this  point  it  has  been  contended  by  Child  that  the  nucleus  is  a  dynamic 
system  capable  of  regenerating  its  lost  parts  and  " producing  a  whole" 
after  amitosis.  But  it  is  a  well  established  fact  that  when  chromosomes 
are  lost  in  abnormal  mitotic  division  they  are  not  regenerated  by  the 
daughter  nuclei  (non-disjunction;  Chapter  XVII). 

In  this  connection  an  experiment  performed  by  Chambers  (1917)  is 
of  interest.  This  investigator  succeeded  in  pinching  the  nucleus  of  an 
animal  egg  into  two  pieces.  The  two  " amitotic ':  nuclei  so  produced 
reunited  upon  touching,  after  which  the  egg  was  fertilized  and  passed 
through  the  early  cleavage  stages  in  the  normal  manner.  It  is  known 
that  the  character  of  these  early  stages  is  largely  independent  of  the 
nuclei  present,  being  the  outgrowth  of  an  organization  already  present 
in  the  egg  cytoplasm.  (See  Chapter  XIV.)  The  later  stages,  in  which 
the  effects  of  the  hereditary  constitution  of  the  nucleus  appear,  were  not 
reached  in  the  present  experiment.  Moreover,  the  entire  chromatin 
outfit  was  present  in  the  reunited  nucleus,  which  is  not  supposed  to  be 
true  of  a  daughter  nucleus  of  an  amitotic  division-.  From  this  experiment, 
therefore,  it  can  only  be  concluded  that  whatever  disturbance  of  the 
spatial  arrangement  of  the  nuclear  elements  may  have  been  caused  by 
the  temporary  separation  of  the  nucleus  into  two  parts,  it  had  no  serious 


OTHER  MODES  OF  NUCLEAR  DIVISION  215 

effect  on  the  nutritive  functions  performed  by  the  nucleus  during  t ln- 
early  cleavage  stages.  Development  did  not  proceed  far  enough  to 
warrant  any  conclusion  regarding  the  effect  upon  the  role  of  the  nucleus 
in  differentiation  and  inheritance. 

Although  what  probably  represents  amitosis  has  been  observed  in 
young  germ  cells,  it  has  not  been  shown  with  certainty  in  any  case  thai 
descendants  of  these  amitotically  dividing  nuclei  become  the  nuclei  of 
normally  functioning  gametes.  To  gain  conclusive  evidence  for  Buch  an 
occurrence  it  would  be  necessary  to  trace  the  descendants  of  t  he  amitol  ic- 
ally  dividing  nuclei  through  to  particular  gametes  or  spores  and  then  to 
note  the  effect  upon  the  individuals  produced  by  them.  This  would  be  a 
matter  of  extreme  experimental  difficulty,  and  not  at  all  possible  in 
most  organisms.  If  it  were  successfully  accomplished  and  the  individuals 
were  found  to  be  normal  in  every  respect,  not  only  in  the  cleavage  stag 
but  throughout  development,  the  revision  of  the  chromosome  theory 
which  various  workers  have  advised  would  at  once  become  necessary. 

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Olive,  E.  W.     1904.     Mitotic  division  of  the  nuclei  of  the  Cyanophyce®.     Beih. 

Bot.  Centr.  18:  9-44.     pis.  1,  2. 
1907.    Cytological  studies  on  Ceratiomyxa.     Trans.  Wis.  Acad.  Bei.  15:  ,.".1  T?:^. 

pi.  47. 
Patterson,  J.  T.     190S.    Amitosis  in  the  pigeon's  egg.     Anat.  An/.  32:  117  125. 

figs.  24. 
Pfeffer,  W.     1899.     Bericht  iiber  amitotische  Kerntheilung.     Ber.  d.  Math. -PL; 

Kl.  d.  Kgl. 'Sachs.  Ges.  Wiss. 


218  INTRODUCTION  TO  CYTOLOGY 

Phillips,  O.  P.     1904.     A  comparative  study  of  the  cytology  and  movements  of  the 

Cyanophyceae.     Contrib.  Bot.  Lab.  Univ.  Pa.  2 :  No.  3. 
vom    Rath,    O.     1891.     Ueber  die   Bedeutung  der  amitotischen   Kerntheilung  im 

Hoden.     Zool.  Anz.  14:  331,  342,  355.     figs.  3. 
Richards,  A.     1909.     On  the  method  of  cell  division  in   Taenia.     Biol.   Bull    17: 

309-326. 
1911.     The  method  of  cell  division  in  the  development  of  the  female  sex  organs  of 

Monezia.     Ibid.  20:  123-178.      8pls. 
Saguchi,  S.     1917.     Studies  on  ciliated  cells.     Jour.  Morph.  29:  217-279.     pis.  1- 4. 
Sakamura,  T.     1920.     Experimen telle  Studien  iiber  die  Zell-  und  Kernteilung  mit 

besonderer  Rucksicht  auf  Form,  Grosse  und  Zahl  der  Chromosomen.     Jour.  Coll. 

Sci.  Imp.  Univ.  Tokyo  39:  pp.  221.     pis.  7. 
Sargant,  E.     1896.     Direct  nuclear  division  in  the  embryosac  of  Lilium  martagon. 

Ann.  Bot.  10:  107-108. 
Schiller,  I.     1909.     Ueber  kunstliche  Erzeugung  "primitiver"  Kernteilungsformen 

bei  Cyclops.    Arch.  Entw.  27 :  560-609.     figs.  62. 
Schurhoff,  P.  N.     1915.     Amitosen  von  Riesenkernen  im  Endosperm  von  Ranun- 
culus acer.     Jahrb.  Wiss.  Bot.  55 :  499-519.     pis.  3,  4. 
1919.     Das  Verhalten  des  Kerns  in  den  Knollchenzellen  von  Podocarpus.     Ber. 

Deu.  Bot,  Ges.  37 :  373-379. 
Scott,  D.  H.     1888.     On  nuclei  in  OsciUatoria  and  Tolypotkrix.     Jour.  Linn.  Soc. 

Bot.  24:  188-192.     pi.  5.     figs.  1-4. 
Strasburger,  E.     1884.     Zur  Entwicklungsgeschichte  der  Sporangien  von  Trichia 

fallax.     Bot.  Zeit.  42:  305-316,  321-326.     pi.  3. 

1907.  Apogamie  bei  Marsilia.     Flora  97:  123-191.     pis.  3-8. 

1908.  Chromosomenzahlen,  Plasmastrukturen,  Vererbungstrager  und  Reduktion- 
steilung.     Jahrb.  Wiss.  Bot.  45.  479-568.     pis.  1-3. 

Tischler,  G.     1900.     Verh.  Naturhist.  Med.  Ver.  Heidelberg  6. 

Trondle,  A.     1912.     Der  Nucleolus  von  Spirogyra  und  die  Chromosomen  hoherer 

Pflanzen.     Zeit.  f.  Bot.  4:  721-747.     pi.  9. 
Wager,  H.     1903.     The  cell  structure  of  the  Cyanophyceae.     (Prelim,  note.)    Proc 

Roy.  Soc.  London  72:  401-408.     figs.  3. 
von  Wasielewski,    W.,    1902,  1903.     Theoretische  u.  Experimen  telle  Beitrage  zur 

Kenntniss  der  Amitose,  I,  II.     Jahrb.  W.  Bot.  38:  377-420,  pi.  7;  39:  581,  606. 

figs.  10. 

Winge,    O.     1912.     Cytological    studies    in    the    Plasmodiophoracese.     Arkiv.    for 

Botanik  12 :  1-39.     pis.  3. 
van  Wisselingh,   C,   1914.     On  the  nucleolus  and  karyokinesis  in  Zygnema.     Rec. 

Trav.  Bot.  Neer.  11:  1-13. 
\  oung,  R.  T.     1913.     The  histogenesis  of  the  reproductive  organs  of  Taenia  pisifor- 

mis.     Zool.  Jahrb.  35:  355-418.     pis.  18-21. 
Zacharias,  E.     1887.     Beitrage  zur  Kenntniss  des  Zellkerns  und  der  Sexualzellen. 
Bot.  Zeit.  45 :  297-304.     pi.  4. 
1890.     Ueber  die  Zellen  der  Cyanophyceen.     Ibid.  48:  1-.     pi.  1. 
1892.     Ueber  die  Zellen  der  Cyanophyceen.     Ibid.  50:  617-624. 
1903.     Jahrb.  Hamb.  Wiss.  Anst.  2]/ 

1907.     Ueber  die  neuere  Cyanophyceen-Literatur.     Bot.  Zeit.  65:  264-287. 
Zukal,    H.     1892.     Ueber  den   Zellinhalt  der   Schizophyten.     (Vorl.    Mitt.)     Ber. 
Deu.  Bot.  Ges.  10:  51-55. 


CHAPTEB  XI 

THE  REDUCTION  OF  THE  CHROMOSOMES 

The  subject  of  chromosome  reduction  is  one  of  the  mosl  important  to 
be  met  with  in  the  study  of  cytology.  Many  of  the  problems,  both  theo- 
retical and  practical,  upon  which  biological  investigators  are  expending 
their  most  intense  efforts  seem  to  be  bound  up  directly  or  indirectly  with 
the  reduction  of  the  chromosomes.  The  essential  feature  of  reductioD  is 
relatively  simple  in  nature,  and  must  be  thoroughly  grasped  in  order  that 
the  discussions  in  the  following  chapters  may  be  intelligible.  The  entire 
process  by  which  reduction  is  accomplished,  on  the  other  hand,  is  very 
complicated  and  extremely  difficult  to  observe  and  interpret  with  any 
degree  of  confidence.  In  spite  of  the  enormous  amount  of  work  already 
done  there  still  exists  much  difference  of  opinion  regarding  some  of  the 
significant  steps  in  the  series  of  changes  undergone  by  the  nuclear  material. 
In  the  present  chapter  a  number  of  these  opinions  will  be  reviewed,  but 
our  main  purpose  will  be  to  make  clear  the  fundamental  feature  of  chro- 
mosome reduction. 

We  have  seen  that  all  the  cells  of  the  body  in  a  given  species  are  char- 
acterized by  the  presence  of  a  certain  number  of  chromosomes  in  their 
nuclei,  and  that  this  number  is  held  constant  throughout  development  by 
an  equational  division  of  every  chromosome  at  every  somatic  mitosis. 
When  we  speak  of  "reduction"  we  ordinarily  refer  to  the  fad  that  at  a 
certain  stage  in  the  life  history  of  the  organism  the  number  of  chromosomes 
is  reduced  one-half.  This  mere  change  in  the  number  of  chromosom 
though  very  important,  is  not  in  itself  the  essential  feature  of  the  reducing 
process,  as  will  be  seen  further  on.  The  whole  number  is  restored  at  the 
time  of  fertilization,  when  two  nuclei,  each  with  the  reduced  number, 
unite.  In  all  organisms  reproducing  sexually  reduction  and  fertilization 
thus  represent  the  two  most  critical  stages  in  the  life  cycle  so  far  as  the 
chromosomes  are  concerned;  hence  the  exhaustive  researches  on  these  two 
processes. 

Discovery. — The  discovery  of  reduction  was  made  by  van  Beneden, 
who  in  1883  announced  that  the  nuclei  of  the  egg  ;md  spermatozoon  of 
A scaris  each  contain  one-half  the  number  <>!'  chromosomes  found  in  the 
body  cells.  Although  van  Beneden  and  other  early  workers  believed  that 
the  change  in  number  was  brought  about  by  the  simple  casting  out  of  half 
the  chromosomes  during  the  growth  of  the  germ  cells,  it  was  soon  shown 
that  this  view  was  incorrect,  and  that  "  reduction  w  effecU  <l  by  n  rearrangi  - 

21 '.» 


220  INTRODUCTION  TO  CYTOLOGY 

.   ment  and  redistribution  of  the  nuclear  substance  without  loss  of  any  of  its 
essential  constituents"  (Wilson  1900,  p.  233). 

In  plants  the  discovery  of  reduction  came  somewhat  later.  Stras- 
burger  in  1888  showed  that  in  angiosperms  the  number  of  chromosomes  in 
the  egg  and  male  nuclei  is  fixed  by  a  reduction  occurring  in  the  mother- 
cells  of  the  embryo  sac  and  pollen  respectively.  This  was  at  once  con- 
firmed by  Guignard  (1889,  1891).  E.  Overton  (1893)  found  that  the 
female  gametophyte  cells  in  the  cycad,  Ceratozamia,  have  half  the  number 
of  chromosomes  found  in  the  cells  of  the  sporophyte.  He  further  sug- 
gested that  reduction  probably  occurs  in  the  sporocytes  in  mosses  and  ferns. 
In  the  liverwort,  Pallavicinia,  Farmer  (1894)  found  the  gametophyte  cells 
to  have  four  chromosomes  and  the  sporophyte  cells  eight.  That  Overton's 
theory  of  a  reduction  in  the  sporocytes  of  bryophytes  and  pteridophytes 
was  correct  was  demonstrated  by  Strasburger  (1894),  who  postulated 
the  occurrence  of  a  periodic  reduction  of  the  chromosomes  in  all  organ- 
isms reproducing  sexually. 

The  Stage  in  the  Life  Cycle  at  which  Reduction  Occurs. — The  reduc- 
tion of  the  chromosomes  is  accomplished  during  the  course  of  two  nuclear 
divisions  which,  since  in  animals  they  have  to  do  with  the  maturing  of  the 
gametes,  early  came  to  be  known  as  the  maturation  divisions.  Because 
of  its  peculiar  character  the  first  of  these  divisions  was  termed  the  hetero- 
typic by  Flemming  (1887),  while  the  second,  which  is  essentially  like  a 
somatic  division,  was  called  the  homceotypic  (sometimes  written  homo- 
typic).  Although  the  essential  act  of  reduction  usually  occurs  at  the  first 
division,  the  entire  process,  to  which  the  name  meiosis  has  been  applied, 
is  of  such  a  nature  that  the  second  division  is  normally  necessary  for  its 
completion.  As  a  result  of  the  two  divisions  the  "  reduced  "  nuclei  or  cells 
are  formed  in  groups  of  four,  or  tetrads,  though  all  members  of  a  tetrad  may 
not  function.  The  point  in  the  life  cycle  at  which  these  divisions  take 
place  in  various  organisms  will  now  be  noted. 

In  animals,  almost  without  exception,  reduction  occurs  at  gameto- 
genesis  (Fig.  77).  In  the  male  those  cells  (spermatogonia)  in  the  testes 
whose  ultimate  descendants  are  to  become  spermatozoa  multiply  by 
divisions  of  the  ordinary  equational  type  until  a  certain  number  are 
produced.  These  cells,  now  called  primary  spermatocytes,  enlarge  a  little 
and  quickly  undergo  two  successive  divisions:  the  first  division  in  each 
is  heterotypic  and  results  in  two  cells  called  secondary  spermatocytes;  the 
second  is  homceotypic  and  divides  the  two  secondary  spermatocytes  into 
four  spermatids,  each  of  which  becomes  transformed  into  a  spermatozoon. 
The  four  spermatozoa  are  therefore  the  immediate  result  of  the  two 
maturation  divisions.  In  the  female  the  situation  is  somewhat  different: 
here  nearly  all  of  the  differentiation  of  the  gamete  is  accomplished  before 
the  nuclear  divisions  bringing  about  reduction  actually  occur.  The 
primary  oocytes  (ovocytes)  are  the  descendants  of  a  number  of  generations 


THE  REDUCTION  OF  THE  CHROMOSOMES 


221 


of  oogonia  (ovogonia).  The  oocyte  usually  while  its  nucleus  is  in  the 
prophases  of  the  first  maturation  division,  enlarges  greatly  ("growth 
period"),  becomes  filled  with  stored  food,  and  develops  the  general  fea- 
tures characterizing  the  egg.  The  oocyte  is  now  called  the  "ovarian  egg/' 
and  it  actually  is  an  egg  in  all  respects  save  one  of  much  importance:  its 
nucleus  still  has  the  full  number  of  chromosomes.  At  a  comparatively 
late  stage,  in  many  cases  even  after  the  spermatozoon  has  entered  the  <rrLr 
at  fertilization,  the  oocyte  nucleus  (germinal  vesicle),  having  passed 
through  some  of  the  prophasic  changes  characteristic  of  the  heterotypic 


-e- 


ANIMAL 


rmriAM    in.»n*Tomt 


division  z 


«Ri    jrunaretiTCk         N — ' 

w 
© 

ru»»  Boat 


^\  S  CAMCTOPtiTTC 


PLANT 


nE.&AS?onocrTi 


DIVISION       I 


JGAMCTOrmU 


pIG  77.— Diagram  showing  the  history  of  the  chromosomes  in  the  ordinary 

life  cycles  of  animals  and  plants. 

mitosis  before  and  during  the  growth  period,  gives  rise  to  a  mitotic  figure 
which  is  often  surprisingly  small  for  the  volume  of  the  aucleus.  The 
spindle  takes  up  a  position  perpendicular  to  the  surface  of  the  cell,  and  a1 
telophase  the  chromosomes  passing  to  the  outer  pole  are  included  in  the 
first  polar  body,  a  small  cell  budded  off  al  this  point.  (See  Fig.  106.)  A 
second  spindle  is  rapidly  formed  about  the  chromosomes  remaining  in 
the  egg  (called  at  this  stage  the  secondary  oocyte)  and  the  second  matura- 
tion mitosis  occurs,  one  daughter  aucleus  being  included  in  the  second 
polar  body.  In  the  course  of  these  two  divisions  chromosome  reduction 
is  accomplished.  The  first  polar  body  may  divide  to  form  two,  thus 
completing  the  tetrad  of  cells  corresponding  to  the  tetrad  of  spermatozoa 
in  the  male.    Although  the  polar  bodies  are  Dormallyfunctionless  they  are 


222  INTRODUCTION  TO  CYTOLOGY 

generally  looked  upon  as  eggs  historically:  the  maturation  divisions 
probably  resulted  formerly  in  a  tetrad  of  eggs,  whereas  now  only  one 
relatively  large  and  highly  differentiated  egg  is  produced  at  the  expense 
of  the  other  three  cells,  which  remain  small  and  functionless. 

Among  the  protozoa  (see  Minchin  1912)  it  has  been  found  that  in 
those  forms  which  appear  to  have  their  chromatin  aggregated  into  no 
definite  number  of  chromosomes,  there  often  occur  two  successive  nuclear 
divisions  suggestive  in  certain  respects  of  maturation  divisions,  a  part  of 
the  products  then  degenerating.  Some  have  regarded  this  as  a  "  casting 
out  of  effete  vegetative  chromatin,"  an  interpretation  which  was  at  one 
time  placed  upon  the  maturation  process  generally.  In  many  cases  this 
"reduction'1  of  the  chromatin  occurs  immediately  prior  to  syngamy 
(sexual  union),1  and  so  agrees  with  reduction  in  higher  forms  in  taking 
place  at  gametogenesis;  but  in  other  cases  it  immediately  follows  syngamy, 
as  in  certain  algse  mentioned  below.  Other  protozoa  have  been  shown  to 
have  a  definite  chromosome  number  which  is  regularly  reduced  in  a  man- 
ner essentially  comparable  to  that  in  the  metazoa. 

In  plants  it  is  among  the  members  of  the  lower  groups  (thallophytes) 
that  a  striking  diversity  is  shown  in  the  stage  of  the  life  cycle  at  which 
reduction  takes  place :  in  the  groups  above  the  thallophytes  it  is  regularly 
accomplished  at  sporogenesis.  In  the  myxomycete,  Ceratio?nyxa,  it 
has  been  shown  by  Olive  (1907)  and  Jahn  (1908)  that  spore  formation  is 
accompanied  by  a  chromosome  reduction.  In  the  green  algse  it  is  in  the 
first  two  divisions  of  the  zygote  (either  a  zygospore  or  a  fertilized  egg) 
that  reduction  occurs:  this  has  been  definitely  established  in  Spirogyra 
(Karsten  1908;  Trondle  1911),  Zygnema  (Kurssanow  1911),  Coleochcete 
(Allen  1905c),  and  Chara  (Oelkers  1916).  In  a  number  of  other  forms, 
such  as  Ulothrix,  (Edogonium,  Sphceroplea,  and  Closterium,  in  which  the 
chromosomes  are  not  well  known,  it  is  probable  that  the  same  condition 
holds,  since  the  zygote  upon  germination  gives  rise  with  considerable 
regularity  to  four  cells;  in  some  cases  {(Edogonium)  these  four  cells  are 
zoospores. 

In  the  brown  alg^e  Cutleria  (Yamanouchi  1912),  Zanardinia  (Yama- 
nouchi),  and  Ectocarpus  (Kylin  1918a)  reduction  occurs  in  connection 
with  zoospore  formation.  In  Fucus,  however,  an  exceptional  condition 
is  found:  here  reduction  takes  place  in  the  antheridium  and  oogonium 
initials,  in  the  first  two  divisions  following  the  one  delimiting  the  stalk 
cell  (Fig.  78,  B).  Since  there  are  only  three  divisions  in  the  oogonium, 
which  thus  produces  eight  eggs,  the  eggs  are  but  one  division  removed 
from  the  four  products  of  the  maturation  mitoses,  a  condition  closely 
approaching  that  in  animals.  That  reduction  in  Fucus  is  associated 
with  gametogenesis  was  inferred  by  Strasburger  (1897)  and  Farmer  and 
Williams  (1898)  and  demonstrated  by  Yamanouchi  (1909). 

1  See  the  cases  of  Actinophrys  sot  and  Amoeba  albida,  Chapter  XII. 


THE  REDUCTION  OF  THE  CHR0M0S0M1 


In  the  red  alg^e  reduction  occurs  in  the  two  divisions  differentiating 
the  nuclei  of  the  tetraspores  when  i  he  Latter  are  presenl  in  t  be  life  history. 
Such  is  the  case  in  Polysiphonia  (Yamanouchi  1906)  I  Fig.  78,  A  .  Gri- 
ffithsia  (Lewis  1909),  and  Corallina  (Yamanouchi).  The  brown  algae 
Dictyota  (Williams  1904)  and  Padina  (Wolfe  L918)  also  conform  to  this 
scheme.  In  Nemalion,  which  has  no  tetraspores,  it  was  long  Buppot 
(Wolfe  1904)  that  reduction  occurs  in  connection  with  carpospore  forma- 
tion, but  Cleland  (1919)  has  recently  shown  thai 
it  takes  place  at  the  time  the  zygote  germinates,  as 
in  so  many  green  alga?.1 

In  the  ascomycetes  reduction  occurs  in  the 
course  of  the  first  two  of  the  three  mitoses  initiated 
by  the  primary  ascus  nucleus  (Figs.  22,  61)  and  re- 
sulting in  the  eight  ascospore  nuclei.  It  was  for  a 
long  time  generally  thought  that  there  were  two 
nuclear  fusions  in  the  life  history — one  in  the  arch  i  cat}) 
and  one  in  the  ascus  (see  p.  290),  and  the  three  di- 
visions in  the  ascus  were  accordingly  regarded  as  a 
process  whose  function  was  to  reduce  the  "  quadri- 
valent" chromosomes  to  the  univalent  condition 
(Harper  1905;  Overton  1906).  Such  a  double  reduc- 
tion was  described  by  Miss  Fraser  (1907,  1908)  for 
Humaria  rutilans:  the  first  mitosis  she  found  to  be 
heterotypic,  the  second  homceotypic,  and  the  third 
"brachymeiotic/'  the  last  bringing  about  a  further 
reduction  by  the  separation  of  the  chromosomes 
into  two  smaller  groups.  This  was  also  reported 
for  Otidea  aurantia  and  Peziza  vesiculosa  (Fraser  and 
Welsford  1908),  Lachnea  stercorea,  Ascobolus  furfura- 
ceas,  and  Humaria  granulata  (Fraser  and  Brooks  1909), 
and  Helvetia  crispa  (Carruthers  1911).  Harper  (1900, 
1905),  although  he  thought  two  fusions  occurred,  found 
no  double  reduction,  holding  rather  that  the  fusion  of  tin-  two  a>ens 
nuclei  and  their  chromsomes  is  so  complete  as  to  render  the  quadrivalent 
character  of  the  latter  entirely  invisible.  Other  investigators  also  find 
no  double  reduction  in  the  ascus.  They  show  rather  that  the  first  two 
mitoses  correspond  to  the  heterotypic  and  homceotypic  mitoses  of  other 
organisms,  and  that  the  third  division  is  purely  vegetative  or  equational 
in  character.     As  instances  may  be  cited  the  work  of  l'aull  I  1905,  1912 

1  For  a  review  of  sexual  reproduction  and  alternation  of  generations  in  tin-  algffi 
see  Bonnet  (1914).  Davis  (1916)  gives  a  convenienl  summary  of  tin-  life  histories 
of  the  red  algie.  Dodge  (1914)  summarizes  and  compares  the  life  histories  of  red 
alga?  and  asconivccU's.  See  Atkinson  I  I'M.",  for  a  complete  review  <>t'  researches  on 
ascomycetes.     For  the  cytology  of  the  yeasts  ruilliermond  1920. 


Fio.   78. 

.1,   prophase  <>f 
heterotypic     division 
in  the  tetrasporocyts 
of  /'  o  I  ys  i  }>  h  i)  ri  ia  . 
(After      Yamanouchi, 
1906.)      Ji,    proph 
of  heterotypic  mit 
in  < oogonium  of  I 
(After  Yatnai 
L909.) 


224 


INTRODUCTION  TO  CYTOLOGY 


on  Hydnobolites,  Neotiella,  and  Laboulbenia,  and  that  of  Claussen  (1912) 
on  Pyronema.  Furthermore,  it  is  becoming  increasingly  apparent  (see  p. 
291)  that  there  is  but  one  fusion  in  the  life  cycle — that  in  the  ascus,  so 
that  the  necessity  for  a  second  reduction  is  removed. 

In  the  basidiomycetes  it  has  been  shown  by  the  researches  of  Juel 
(1898),  Maire  (1905),  Guilliermond  (1910),  Kniep  (1911,  1913),  Levine 
(1913),  and  others  on  the  hymenomycetes,  and  by  those  of  V.  H.  Black- 
man  (1904),  Dietel  (1911),  Fitzpatrick  (1918),  and  others  on  the  rusts,1 
that  reduction  occurs  in  the  two  mitoses  giving  rise  to  the  four  basidio- 


Fig.  79. — Sexual  fusion  and  maturation  divisions  in  the  basidium  of 

Nidularia  piriformis . 

a,  two  sexual  nuclei  about  to  unite,  b,  prophase  of  heterotypic  division  in  fusion 
nucleus,  c,  heterotypic  mitosis,  d,  homceotypic  mitosis,  e,  the  four  basidiospore 
nuclei.      X  1800.     (After  Fries,  1911.) 


spore  nuclei  (Fig.  79) .  As  in  the  ascomycetes,  it  thus  follows  immediately 
upon  the  nuclear  fusion:  in  the  basidium  in  hymenomycetes  and  in  the 
teleutospore  in  rusts.  An  exception  is  reported  in  the  case  of  Hygro- 
phorus  conicus,  in  which  Fries  (1911)  finds  in  the  basidium  neither  a 
nuclear  fusion  nor  a  reduction. 

In  the  bryophytes  reduction,  so  far  as  known,  is  universally  brought 
about  by  the  two  mitoses  which  differentiate  the  four  nuclei  of  each  spore 
tetrad.  It  was  at  one  time  reported  (van  Leeuwen-Reijnvaan  1907) 
that  in  Polytrickum  there  is  a  second  reduction  at  spermatogenesis  and 
oogenesis:  the  sporophyte  was  said  to  have  12  chromosomes,  the  spore 
and  gametophyte  six,  and  the  gametes  three.  This  double  reduction 
was  thought  to  be  compensated  for  by  the  fusion  of  the  ventral  canal 
cell  with  the  egg,  raising  the  number  in  the  latter  to  six,  in  combination 
with  the  entrance  of  two  sperms  into  the  egg  at  fertilization,  making  the 
sporophytic  number  12.  This  interpretation  has  been  shown  to  be 
false  by  both  Vandendries  (1913)  and  Walker  (1913),  who  find  the  life 
cycle  normal  in  every  respect:  a  reduction  from  12  to  6  occurs  at  sporo- 
genesis  but  no  second  reduction  follows  at  gametogenesis. 

1  A  summary  of  researches  on  rusts  is  given  by  Maire  (1911).  A  list  of  numbers  of 
nuclei  in  the  cells  of  basidiomycetes  is  given  by  Levine  (1913). 


THE  REDTCTIOX  OF  THE  CHROMOSOMES 


In  vascular  plants  reduction  in  all  normal  life  cycles  in  both  homo- 
sporous  and  heterosporous  forms  occurs  uniformly  in  the  divisions  differ- 
entiating the  spore  tetrads  (Fig.  77).  The  sporocytes,  particularly  the 
microsporocytes  ("pollen  mother-colls"),  of  the  higher  plants  have  long 
been  favorite  objects  for  the  study  of  reduction.  Since  the  gametophyte 
generation  in  the  higher  plants  is  so  abbreviated,  reduction  closely  pre- 
cedes fertilization  in  these  forms.  In  the  ordinary  angiosperm  embryo 
sac  in  which  the  eight  nuclei  are  derived  from  a  single  megaspore  of  the 
tetrad,  the  egg  nucleus  is  removed  from  the  product  of  reduction  (mega- 
spore nucleus)  by  only  three  mitoses.  In  some  cases,  of  which  Lilium 
is  the  best  known  example,  walls  fail  to  form  be- 
tween the  four  megaspore  nuclei  (Fig.  80,  B), 
leaving  them  in  a  common  cavity  (embryo  sac) 
where  they  undergo  but  one  further  division  to 
produce  the  eight  nuclei  of  the  female  gameto- 
phyte. The  egg  here  is  consequently  removed 
from  the  product  of  reduction  by  a  single 
mitosis.  In  one  known  case,  Plumbagella 
(Dahlgren  1915),  the  four  reduced  nuclei, 
formed  as  in  Lilium,  divide  no  further,  one  of 
them  functioning  directly  as  the  egg  nucleus. 
Here,  therefore,  the  condition  characteristic  of 
animals  has  been  reached:  the  gamete  nucleus  is 
itself  the  direct  product  of  reduction,  and  the 
haploid  generation  usually  produced  by  the 
spore  is  eliminated.  The  male  gametophyte 
also  has  undergone  much  abbreviation  in  higher 

plants,  but  the  male  nucleus  is  still  removed  from  the  reduction  product 
(microspore  nucleus)  by  two  mitoses.  In  no  known  case  does  the  micro- 
spore nucleus  function  directly  as  a  gamete  nucleus. 

The  term  gonotokont  was  introduced  by  Lotsy  (1904)  to  designate 
any  cell,  whatever  its  origin  or  position  in  the  life  cycle,  in  which  the 
reduction  process  is  initiated.  In  animals  the  gonotokonts  arc  therefore 
the  primary  spermatocyte  and  the  primary  oocyte.  In  most  green  algffl 
the  gonotokont  is  the  zygote;  in  the  red  alga'  it  is  usually  the  tetrasporo- 
cyte;  in  the  ascomycetes  it  is  the  ascus;  in  the  basidiomyeetes  il  Is  the 
basidium;  and  in  the  bryophytes  and  vascular  plants  i1  is  the  Bporocyfc 
the  microsporocytc  and  megasporocyte  in  the  case  of  heterosporous  forms. 

The  Meaning  of  Reduction.— In  order  that  the  true  meaning  of  reduc- 
tion may  be  appreciated  it  will  be  necessary  to  indicate  the  mam  points 
of  a  theory  first  suggested  by  Roux  I L883)  and  later  developed  particu- 
larly by  Weismann  (1887, 1891, 1892).  It  had  been  believed  by  the  earlier 
workers  that  reduction  was  merely  a  process  whose  function  was  'to 
prevent  a  summation  through  fertilization  of  the  nuclear  mass  and  of 


In,.      8  0  — M  e  k'  :i  M>  >>  r  «' 
tetrads  in   Angiosperms. 

.1 ,  tetrad  of   walled   cells 

in  Phy.soslegia  viTffiniana', 
formation  of  two  upper 
ones  just  being  completed. 
X  462.  {After  sharp,  1911.) 
B,  tetrad  of  megaspore 
nuclei  in  Lilium   canad* 


15 


226  INTRODUCTION  TO  CYTOLOGY 

the  chromatic  elements"  (Hertwig  1890).  But  the  chromatic  mass  is 
actually  quartered  at  reduction,  whereas  the  number  of  chromosomes  is 
halved.  Moreover,  great  changes  in  nuclear  volume  occur  with  no 
change  in  the  number  of  chromosomes.  This  careful  guarding,  so  to 
speak,  of  the  chromosome  number  was  siezed  upon  as  a  most  significant 
fact  by  Roux,  who  "argued  that  the  facts  of  mitosis  are  only  explicable 
under  the  assumption  that  the  chromatin  is  not  a  homogeneous  substance, 
but  differs  qualitatively  in  different  regions  of  the  nucleus;  that  the 
collection  of  the  chromatin  into  a  thread  and  its  accurate  division  into 
two  halves  is  meaningless  unless  the  chromatin  in  different  regions  of  the 
thread  represents  different  qualities  which  are  to  be  divided  and  dis- 
tributed to  the  daughter  cells  according  to  some  definite  law.  He  urged 
that  if  the  chromatin  were  qualitatively  the  same  throughout  the  nucleus, 
direct  division  would  be  as  efficacious  as  indirect,  and  the  complicated 
apparatus  of  mitosis  would  be  superfluous."1  Upon  this  conception 
Weismann  based  his  remarkable  theory,  the  starting  point  of  which  was 
"the  hypothesis  of  De  Vries  that  the  chromatin  is  a  congeries  or  colony  of 
invisible  self-propagating  vital  units  or  biophores,  somewhat  like  Darwin's 
'gemmules,'  each  of  which  has  the  power  of  determining  the  development 
of  a  particular  quality.  Weismann  conceives  these  units  as  aggregated 
to  form  units  of  a  higher  order  known  as  'determinants,'  which  in  turn  are 
grouped  to  form  'ids,'  each  of  which  ...  is  assumed  to  possess  the 
complete  architecture  of  the  germ-plasm  characteristic  of  the  species. 
The  'ids'  finally,  which  are  identified  with  the  visible  chromatin-granules, 
are  arranged  in  linear  series  to  form  'idants'  or  chromosomes.  It  is 
assumed  further  that  the  'ids'  differ  slightly  in  a  manner  corresponding 
with  the  individual  variations  of  the  species,  each  chromosome  therefore 
being  a  particular  group  of  slightly  different  germ-plasms  and  differing 
qualitatively  from  all  the  others. 

"We  come  now  to  the  essence  of  Weismann's  interpretation.  The 
end  of  fertilization  is  to  produce  new  combinations  of  variations  by  the 
mixture  of  different  ids.  Since,  however,  their  number,  like  that  of  the 
chromosomes  which  they  form,  is  doubled  by  the  union  of  two  germ- 
nuclei,  an  infinite  complexity  of  the  chromatin  would  soon  arise  did  not 
a  periodic  reduction  occur.  Assuming,  then,  that  the  'ancestral  germ- 
plasms'  (ids)  are  arranged  in  a  linear  series  in  the  spireme  thread  or  the 
chromosomes  derived  from  it,  Weismann  ventured  the  prediction  (1887) 
that  two  kinds  of  mitosis  would  be  found  to  occur.  The  first  of  these  is 
characterized  b}r  a  longitudinal  splitting  of  the  thread,  as  in  ordinary 
cell-division,  'by  means  of  which  all  the  ancestral  germ-plasms  are 
equally  distributed  in  each  of  the  daughter-nuclei  after  having  been 
divided  into  halves.'  This  form  of  division,  which  he  called  equal 
division    (Aequationstheilung),    was  then   a   known  fact.     The  second 

^his  and  the  following  quotations  are  from  Wilson  (1900,  pp.  245-246). 


THE  REDUCTION  OF  THE  CHROMOSOMES  227 

form,  at  that  time  a  purely  theoretical  postulate,  he  assumed  to  be  of 
such  a  character  that  each  daughter-nucleus  should  receive  only  half  the 
number  of  ancestral  germ-plasms  possessed  by  the  mother-nucleus. 
This  he  termed  a  reducing  division  (Reduktionstheilung),  and  suggested 
that  this  might  be  effected  either  by  a  transverse  division  of  the  chromo- 
somes, or  by  the  elimination  of  entire  chromosomes  without  division.  By 
either  method  the  number  of  'ids'  would  be  reduced;  and  Weismann 
argued  that  such  reducing  divisions  must  be  involved  in  the  formation 
of  the  polar  bodies,  and  in  the  parallel  phenomena  of  spermatogenesis.'1 

Reduction  in  Weismann's  sense,  then,  is  a  reduction  of  the  number  of 
kinds  of  germ-plasm  or  ancestral  hereditary  qualities  present,  this 
reduction  being  brought  about  by  means  of  a  redistribution,  half  of  the 
qualities  to  one  daughter  nucleus  and  the  remainder  to  the  other  daughter 
nucleus.  The  change  in  the  number  of  chromosomes  is  a  consequence  of 
the  manner  in  which  this  redistribution  is  accomplished,  as  we  shall  see. 

Interpretations  Based  on  Weismann's  Theory. — As  would  be  ex- 
pected, there  were  announced  certain  interpretations  of  chromosome  be- 
havior based  on  Weismann's  idea.  Several  cytologists  thought  that 
they  found  the  chromsomes  actually  dividing  transversely  at  one  or  the 
other  of  the  two  maturation  mitoses.  This  interpretation,  however, 
proved  to  be  incorrect.  Much  light  w7as  thrown  on  the  problem  when 
Henking  (1891),  Rtickert  (1891,  etc.),  Haecker  (1890-9),  vom  Rath 
(1892-3),  and  others  showed  that  the  double  chromosomes  appearing  in 
the  reduced  number  on  the  spindle  at  the  first  maturation  mitosis  are  not 
split  chromosomes  like  those  seen  in  somatic  divisions,  but  are  pair-  oi 
chromosomes,  or  hivalent  chromosomes,  each  arising  b}r  an  end-to-end 
conjugation  (synapsis)  of  two  somatic  chromosomes.  The  two  parts  oi 
each  bivalent  then  separate  at  the  first  or  second  maturation  division, 
the  entire  chromosomes  thus  being  segregated  into  two  groups,  each 
with  the  reduced  number.  Thus  it  appeared  unnecessary  thai  a  single 
chromosome,  representing  a  linear  series  of  different  qualities,  should  be 
transversely  divided  in  order  for  Weismannian  reduction  to  occur:  it 
was  only  necessary  to  assume  that  the  whole  chromosomes  differ  qualita- 
tively from  one  another,  so  that  when  the  two  members  of  a  bivalent 
pair  separate  there  would  be  a  segregation  of  different  qualities.  It  is  in 
the  light  of  this  bivalent  chromosome  conception  thai  we  are  to  interpret 
the  many  early  reports  of  a  transverse  division  of  the  chromosome  during 
maturation.  What  was  called  a  transverse  division  was  merely  tin- 
separation  of  two  entire  chromsomes  placed  end-to-end. 

A  number  of  workers  soon  found  that  in  many  cases  there  Is  nothing 
even  simulating  a  transverse  division,  either  of  single  chromosomes  or  <>t 
bivalent  pairs,  but  that  both  maturation  divisions  are  apparently  longi- 
tudinal (Flemming,  Brauer  1S93,  Moore  L896,  Meves  L896,  Gregoire, 
etc.).     Howt,  then,  is  there  any  Weismannian  reduction  if  there  is  neither 


228 


INTRODUCTION  TO  CYTOLOGY 


a  transverse  division  of  the  chromosome  nor  a  transverse  separation  of 
bivalents?  Montgomery  (1901),  von  Winiwarter  (1900),  Sutton  (1902), 
Boveri  (1904),  and  a  number  of  others  showed  that  here  the  chromo- 
somes conjugate  side-by-side  rather  than  end-to-end.  Thus  when  they 
separate  there  is  an  appearance  of  a  longitudinal  division,  but  reduction 
is  nevertheless  accomplished,  since  entire  somatic  chromosomes  suppos- 
edly qualitatively  different,  and  not  the  longitudinal  halves  of  split 
chromosomes,  are  separating.  The  appearance  of  a  longitudinal  division 
may  also  be  present  after  an  end-to-end  conjugation,  for  the  two  members 
may  bend  around  to  a  side-by-side  position  before  finally  separating. 


SOMATIC       MITOSIS 
—   EQWATIONAI-- 


<:-— J> 


HETEROTYPIC         MIT0SI3 
-  REDUCTIONAU   - 


Fig.  81. — Diagram  showing  essential  difference  between  somatic  and  heterotypic 

mitoses. 

As  a  matter  of  fact,  the  maturation  divisions  in  nearly  all  cases,  especially 
those  studied  by  botanists,  are  both  longitudinal  in  appearance.  End- 
to-end  conjugation  later  came  to  be  called  telosynapsis  or  metasyndese, 
and  side-by-side  conjugation  parasynapsis  or  parasyndese. 

Somatic  and  Heterotypic  Mitoses  Compared. — Before  taking  up  a 
more  detailed  account  of  the  process  of  reduction  as  it  has  been  described 
by  various  investigators  it  is  of  the  utmost  importance  to  fix  clearly  in 
mind  the  essential  difference  between  somatic  and  heterotypic  mitosis 
in  order  to  realize  what  constitutes  the  cardinal  feature  of  reduction,  and 
thereby  to  detect  the  significant  points  of  the  various  theories.  This 
essential  difference  is  illustrated  in  Figs.  81  and  82.  In  a  somatic  or 
vegetative  mitosis  every  chromosome  is  split  into  two  exactly  similar  longi- 
tudinal halves  which  are  distributed  to  the  two  daughter  nuclei.  The  daughter 
nuclei  are  therefore  like  each  other  and  like  the  mother  nucleus  in  the  quality 


THE  REDUCTION  OF  THE  CHROMOSOMES 


229 


of  their  substance.  In  the  heterotypic  mitosis,  the  first  of  the  two  matura- 
tion mitoses,  the  chromosomes  conjugate  two  by  two  during  the  prophase  to 
form  the  reduced  number  of  bivalent  chromosomes,  which  take  their  place 
on  the  spindle.  The  members  of  each  pair,  which  are  supposi  d  to  diffi  r 
qualitatively  from  each  other,  separate  and  pass  to  the  two  daughter  nuclei. 
These  nuclei  are  therefore  qualitatively  unlike  each  other,  having  different 
members  of  the  full  chromosome  group;  and  also  unlike  the  mother  nucL  ust 
since  each  of  them  has  only  half  as  many  chromosomes  as  the  latter.  The 
second  maturation  mitosis  (not  shown  in  the  diagrams)  is  essentially  a 
vegetative  mitosis  in  most  cases:  each  chromosome  splits  longitudinally 


SOMATIC         MITOSIS 
•-    EQUATlONAL   - 


<W\ 


\VW/' 


HETEROTYPIC       MITOSIS 
—      tteiUCTIONAL     — 


A 


/•* 


Fig.  82. — Diagram  showing  essential  difference  between  somatic  and  hetero- 
typic mitoses. 

and  the  halves  are  distributed  to  the  daughter  nuclei.     The  four  nuclei, 
and  consequently  the  four  cells,  resulting  from  the  two  maturation  divi- 
sions are  therefore  of  two  kinds:  two  of  them  have  half  of  the  chromos<  »n . 
of  the  original  nucleus  and  the  other  two  have  the  remaining  ones. 

Assuming,  then,  that  the  chromatin  of  the  nucleus  represents  the 
principal  physical  basis  of  inheritance  (see  Chapter  XIV),  reduction  is 
essentially  this:  a  reduction  in  the  number  of  kinds  of  hereditary  units  by 
the  separation  and  distribution  of  qualitatively  diffi  n  nt  masses  of  chromatin 
to  different  cells  and  eventually  into  different  hereditary  Urn  s,  ratio  r  than  an 
equational  division  and  distribution  of  all  the  qualitu  8  as  in  somatic  mitosis. 
As  has  already  been  stated,  the  change  in  the  Dumber  of  chromosomes 
("numerical  reduction")  is  a  consequence  of  the  method  by  which  this 
qualitative  reduction  is  brought  about,  this  method  being  the  distribution 
of  entire  chromosomes,  each  representing  one  or  more  part  icular  qualities, 
to  different  cells. 


230  INTRODUCTION  TO  CYTOLOGY 

Another  important  feature  of  the  reduction  process  should  be  noted 
before  proceeding  further.  In  many  cases,  chiefly  among  animals,  the 
chromosomes  appearing  on  the  spindle  of  the  first  maturation  mitosis  are 
not  merely  double,  but  quadruple.  This  is  due  to  the  fact  that  each  of  the 
conjugating  chromosomes  is  already  longitudinally  split,  giving  the 
bivalent  chromosome  the  form  of  a  chromosome  tetrad  (not  to  be  confused 
with  tetrads  of  cells  or  nuclei).  The  four  constituents  of  the  chromosome 
tetrad  are  known  as  chromatids,  and  are  distributed  by  the  two  matura- 
tion mitoses  to  the  four  resulting  cells.  It  is  thus  seen  that  in  the  case  of 
chromosome  tetrads  the  lines  of  separation  for  both  maturation  mitoses 
are  marked  out  in  the  prophase  of  the  first.  It  should  be  borne  in  mind 
that  one  of  these  lines  represents  a  plane  of  chromosome  conjugation, 
and  the  other  a  plane  of  true  longitudinal  splitting.  When  the  chroma- 
tids separate  along  the  conjugation  plane  reduction  occurs,  whether  this 
be  at  the  first  or  second  mitosis,  and  when  separation  along  the  plane  of 
splitting  occurs  the  mitosis  is  equational,  as  in  somatic  division. 

MODES  OF  CHROMOSOME  REDUCTION 
In  all  cytology  there  is  scarcely  a  subject  upon  which  there  has  been 
entertained  so  great  a  variety  of  opinion  as  upon  the  question  of  the 
exact  behavior  of  the  chromosomes  during  the  meiotic  phases.  Entirely 
aside  from  the  theoretical  interpretations  placed  upon  the  process  of 
maturation,  cytologists  have  yet  failed  to  arrive  at  any  universally 
accepted  conclusion  regarding  all  the  structural  changes  which  occur. 
This  diversity  of  opinion  is  due  in  part  to  the  complexity  of  the  process 
and  the  difficulty  of  interpreting  its  various  stages,  some  of  which  fail 
to  stand  out  clearly  in  preparations  made  by  our  available  methods. 
On  the  other  hand,  a  great  variety  of  organisms  have  been  studied,  and 
these  undoubtedly  differ  considerably  in  the  details  of  the  reduction 
process,  so  that  agreement  in  all  particulars  is  not  to  be  expected.  The 
attempt  has  too  often  been  made  to  apply  universally  an  interpretation 
founded  upon  a  study  of  one  or  two  organisms.  Certain  essential  fea- 
tures of  meiosis  may  be  expected  to  show  close  agreement  in  all  organisms 
reproducing  sexually,  as  Strasburger  pointed  out,  but  it  is  evident  that 
there  is  no  full  correspondence  as  regards  the  exact  manner  in  which  the 
essential  changes  are  accomplished.  In  the  following  pages  are  given 
brief  descriptions  of  a  few  representative  interpretations  advanced  by 
various  cytologists.1 

The  two  interpretations  of  reduction  which  have  been  most  conspicu- 
ous in  the  literature  of  recent  years  are  diagrammed  in  Figs.  83  and  89. 

1  No  attempt  can  be  made  in  a  work  of  this  scope  to  give  a  complete  summary 
and  classification  of  all  the  interpretations  that  have  been  put  upon  the  maturation 
phenomena.  Only  enough  will  be  presented  to  afford  a  starting  point  for  a  study  of 
this  complex  subject.  For  a  review  and  criticism  of  all  views  expressed  up  to  1910 
see  Gregoire's  two  invaluable  works  (1905,  1910).  A  useful  list  of  works  on  somatic 
and  heterotypic  mitosis  in  angiosperms  is  given  by  Picard  (1913). 


THE  REDUCTIOX  OF  THE  CHR0M0S0M1 


231 


Nearly  all  of  the  accounts  of  reduction  now  appearing,  especially  tho 
given  by  botanists,  conform  in  general  to  one  or  the  other  of  these  two 
schemes,  though  they  vary  greatly  in  detail.  Both  theories  have  been 
upheld  by  competent  observers,  and  it  may  be  possible  thai  both  modes 
of  reduction  actually  occur;  but  the  same  objects  have  been  so  differently 
described  by  the  two  opposing  schools  that  it  seems  very  probable  thai 
interpretation  is  chiefly  responsible  for  the  persistent  diversity  of  opinion. 
For  convenience  the  two  theories  will  be  referred  to  aa  Schenu  A  and 
Scheme  B. 


REST 


SYNIZESIS 


PACHYNEMA 


LtPTONEMA 


STBEPSINEMA 


ZYCONEMA 


ClAKIMCSIS 


MCTEHOT-rni   mioiu 


nanoT^nc  mitosis 


Fig.   83. — The  method  of  chromosome  reduction  according  t<>    9W  1. 

Explanation  in  text. 

Scheme  A. — The  first  of  the  two  main  interpretations  of  reduction 
came  into  prominence  in  1900  and  shortly  after,  when  von  Winiwarl 
(1900),  Gregoire  (1904,  1907,  1909),  A.  and  K.  E.  Schreiner  (1904    L908  . 
and  Berghs  (1904,  1905)  applied  it  to  the  phenomena  observed  by  them 
in  several  animals  and  plants.     It-  essential  points  are  a-  follows    Figs. 

83-88) : 

At  the  beginning  of  the  heterotypic  prophase  the  nuclear  reticulum, 
without  breaking  down  into  such  distinct  elementary  aets  or  alveolar 
units  as  are  seen  in  the  somatic  prophase,  take-  the  form  of  long  slender 
threads  (leptothic  or  leptonema  stage).1     During  the  very  early  prophi 

1  The  terms  Uptotene,  synaptene,  pachytene,  and  <liplot*,<  were  proposed  by  von 
Winiwarter   (1900);  leptonema,    zygotene,   pachynema,  and  stre]  bj    Gregoire 

(1907);  amphitene  by  Janssens  (1905  ;   itrepsUene  by   Dixon     I"""  :  d  tis  by 

Haecker  (1897) ;  synapsis  by  Moore    L896  ;    .  b]  McC  ing    1906  ;andf* 

by  Fanner  and  Moore  (1905).     The  terms  ending  in  -/.'/.<    arc  ordinarily  used  as 
adjectives. 


232 


INTRODUCTION  TO  CYTOLOGY 


these  threads  conjugate  in  pairs  side-by-side  (parasynaptically ;  para- 
syndetically).  The  association  does  not  take  place  at  all  regions  of  the 
threads  at  once:  it  begins  at  one  or  two  points,  commonly  at  one  end, 


H 


Fig.  84. — Reduction  in  sporocyte  of  Nephrodium. 
A,  leptonema.     B,  recovery  from  synizesis;  parallel  conjugation  consummated  during 
symzesis.      C,   pachynema.     D,   second   contraction  of  bivalent  spireme.     E,   diakinesis. 
F,   anaphase   of  heterotypic  mitosis.     G,  interkinesis.     H,  homceotypic  mitosis.     I,  two 
of  the  four  spore  cells.      (After  Yamanouchi,  1908.) 

and  gradually  involves  all  portions,  so  that  at  stages  when  the  process  is 
yet  incomplete  the  two  threads  may  be  closely  paired  at  some  points  and 
widely  divergent  at  others,  giving  an  appearance  very  unlike  that  of  the 
halves  of  a  longitudinally  split  chromosome  in  a  somatic  cell.     This  is 


THE  REDUCTION  OF  THE  CHROMOSOMES 


known  as  the  zygotene,  zygonema,  synapthw,  or  amphiteru  stag  Usually 
before  the  union  is  complete  the  nucleus  enlarges  somewhal  and  the 
threads  contract,  forming  a  tight  knot  at  one  side  of  the  nucleus.  Tin- 
stage,  formerly  called  synapsis  (see  p.  255),  is  now  more  properly  known 
synizesis.  The  pairing  threads  conic  into  very  close  association  during 
synizesis,  which,  though  variable  in  time,  usually  ensues  a1  about  this 
stage.  When  the  closely  paired  threads  recover  from  the  synizesis  con- 
traction they  extend  more  uniformly  throughout  the  nucleus  ("open 
spireme"),  and  are  now  seen  to  be  muchithickera(/;r/rA////m  ;  pachym  ma  . 
In  many  cases  they  may  again  contract  into  a 
loose  knot  with  loops  extending  from  it  ("  second 
contraction").  As  they  continue  to  decrease  in 
length  and  increase  in  diameter  the  members  of 
each  pair  twist  more  or  less  tightly  about  each 
other  for  a  short  time  (strepsitene;  strepsineina; 
diplotene).  Eventually  they  become  very  short 
and  thick,  and  the  various  pairs  (gemini; 
bivalent  chromosomes),  present  in  the  haploid 
or  reduced  number,  lie  scattered  throughout 
the  nucleus  (diakinesis) .  The  two  components 
of  each  geminus  may  now  separate  slightly  at 
one  or  both  ends  or  at  the  middle,  which  gives 
them  the  form  of  Ys,  Vs,  X.s,  and  Os.  The 
bivalent  chromosomes  are  now  fully  formed  and 
ready  to  take  their  places  on  the  spindle,  which 
soon  forms. 

In  the  case  of  the  animal  egg  the  " growth 
period"  introduces  a  complication.  In  the 
sporocytes  of  plants  and  the  spermatocytes  of 
animals  >there  is  some  enlargement  of  the  cell 
and  nucleus  during  the  stages  just  describee  I, 
but  the  chromosomes  pass  directly  from  the 
strepsinema  stage  to  diakinesis.  During  the 
relatively  enormous  growth  of  the  oocyte 
the  chromosomes,  which  have  usually  reached  the  strepsinema 
when  the  enlargement  begins,  become  greatly  modified  in  form. 
Their  achromatic  framework  takes  the  form  of  fine  threads  extending 
out  in  all  directions,  giving  the  chromosome  an  irregular  brush- 
like form  (Fig.  86,  C,  D),  while  the  chromatic  substance  either  may 
flow  into  the  nucleolus,  leaving  the  chromosome  framework  uncolored  and 
very  difficult  to  observe  or  by  loss  of  its  staining  capacity  through  chem- 
ical change  it  may  disappear  from  view  completely,  is  the  growth 
period  comes  to  an  end,  however,  the  original  staining  capacity  returns 
and  the  chromosomes  again  assume  the  compact  form  and  pass  into  the 
diakinesis  stage. 


Fig.    85.-  -Parasynapsis  in 

Phrynntf  ttix   ma  . 

.1.   Leptonema;    x, 
chromosome.     B,  conjuga- 
tion of  "  chromosome    l . 
Portions   of   other  uncon- 
jugated  threads    ami    one 
Hi  her  bivalent  also  pn 

i  n   Beef  ion.      X    21 

After  Went  cA,  1911 

on    the    other    hand, 


234 


INTRODUCTION  TO  CYTOLOGY 


In  the  case  of  most  animals,  and  apparently  in  certain  plants  also, 
the  split  which  is  to  function  in  the  homceotypic  mitosis  may  develop  dur- 
ing diakinesis  or  even  much  earlier,  the  result  being  the  formation  of 
chromosome  tetrads.  This  introduces  another  element  of  complication 
which  will  be  touched  upon  later  (p.  243). 


i 

Fig.    86. 

A,  parasynapsis  in  Allium  fistulosum.  B,  parasynapsis  in  Osmunda  regalis.  C,  nucleus 
of  oocyte  of  Scyllium  canicula  (Selachian)  in  "growth  stage."  D,  single  chromosome  in 
growth  stage,  showing  the  fine  subdivision  of  its  substance.  (A  and  B  after  Gregoire,  1907, 
C  and  D  after  Marechal  1907.) 

The  diakinesis  stage  is  terminated  by  the  dissolution  of  the  nuclear 
membrane  and  the  formation  of  the  spindle,  upon  which  the  bivalent 
chromosomes,  whether  secondarily  split  or  not,  now  become  arranged. 
Because  of  the  peculiar  form  and  consistency  of  the  heterotype  chromo- 
somes the  mitotic  figure  presents  a  striking  contrast  in  appearance  to  the 
ordinary  figure  of  somatic  cells.  This  is  especially  true  as  the  chromo- 
somes are  drawn  into  various  curious  shapes  as  their  anaphasic  separation 
begins.  The  two  univalent  components  of  each  bivalent  chromosome 
eventually  become  free  from  each  other  and  pass  to  the  two  daughter 
nuclei,  bringing  about  reduction.     During  the  anaphase  the  separating 


THE  REDUCTION  OF  THE  CHROMOSOMES 


J^P 


Fig.  87. — Nuclei  from  microsporocytes  of   Viria  faba,  showing  parasynapsis. 
Synigesis    beginning    in    No.    6.      X  1900. 


Fig.   88. — Heterotypic   prophases   In   spermatocyte   of    Tomopteris 

A,  pairing  of  leptotene  threads  beginning.  />'.  pairing  complete  in  some  threads  and 
only  beginning  in  others.  C,  conjugation  complete;  pachynema  stage.  D,  resplitting 
of  pachytene   threads   (separation  of  conjugated   chromosomes  l  l.  mul   K 

Schreiner,  1905.) 


236 


INTRODUCTION  TO  CYTOLOGY 


univalents,  if  not  already  double,  rapidly  develop  a  longitudinal  split,  in 
some  cases  even  before  they  are  entirely  free  from  each  other.  The 
resulting  halves  tend  to  open  out  along  this  split;  chromosomes  being 
drawn  endwise  to  the  poles  thus  take  the  form  of  simple  Vs,  while  those 
to  which  the  fibers  are  attached  at  the  middle  appear  as  double  Vs.  After 
reaching  the  poles  the  split  chromosomes  begin  the  reconstruction  of  the 
daughter  nuclei.  As  a  rule  this  does  not  proceed  very  far,  since  the 
homceotypic  mitosis  follows  very  quickly  upon  the  heterotypic.  Well 
organized  daughter  nuclei  are  often  formed,  whereas  in  the  animal  egg 
there  may  be  no  reconstruction  whatever,  the  daughter  chromosomes  of 
the  first  mitosis  at  once  taking  their  places  on  a  newly  formed  spindle  for 
the  second  mitosis. 

In  the  homceotypic  mitosis  the  chromosomes,  if  there  has  been  an  inter- 
vening interkinesis  of  any  length,  usually  appear  much  longer  and  thin- 
ner than  in  the  heterotypic  mitosis,  and  separate  along  the  longitudinal 
line  of  fission  seen  in  the  preceding  anaphase.  The  homceotypic  mitosis 
is  therefore  equational  in  character,  and  differs  from  an  ordinary  somatic 
mitosis  only  in  the  number  of  its  chromosomes  and  the  precocity  of  their 
splitting.  In  each  of  the  four  nuclei  resulting  from  the  two  maturation 
mitoses  there  is  now  the  haploid  number  of  univalent  chromosomes,  and 
meiosis  is  complete. 

The  foregoing  interpretation  of  reduction  has  been  widely  accepted 
from  the  first  by  both  botanists  and  zoologists.  The  following  is  a  partial 
list  of  works  in  which  it  has  been  described. 


Gregoire 

1904,  '07 

Berghs 

1904,  '05 

Rosenberg 

1905,  '07, 

Allen 

19056c 

J.  B.  Overton 

1905,  '09 

Strasburger 

1905,  '07, 

Miyake 

1905 

Tischler 

1906 

Cardiff 

1906 

Lagerberg 

1906,  '09 

Yamanouchi 

1906,  '08, 

Martins  Mano 

1909 

Lundegardh 

1909,  '14 

Frisendahl 

1912 

McAllister 

1913 

Schneider 

1913,  '14 

Weinzieher 

1914 

Sakamura 

1914 

de  Litardiere 

1917 

Plants 

Lilium,  Allium,  Osmunda 

Allium,  Drosera,  Helleborus,  etc. 
'08,  '09    Drosera,  Compositse 

Lilium,  Coleochcete 

Thalictrum,  Calycanthus,  Richardia 
'08,  '09    Lilium,  Galtonia,  etc. 

Lilium,    Funkia,    Iris,    Allium,    Trades- 
cantia,  Galtonia 

Ribes 

Acer,  Salomonia,  Botrychium,  Ginkgo 

Adoxa 
'10  Polysiphonia.  Nephrodium,  Osmunda 

Funkia 

Trollius 

Myricaria 

Smilacina 

Thelygonium 

Xyris 

Vicia 

Polypodium 


THE  REDUCTION  OF  THE  CHROMOSOMES 


23' 


Animals 


von  Winiwarter 
Marechal 


1900 

1904,  '05,  '07 


A.  and  K.  E.  Schreiner  1906,  '07,  '08 


Lerat 

1905 

Deton 

1908 

Gregoire 

1909 

Janssens 

1905, 

'09 

Janssens  et  Willems 

1909 

Schleip 

1906, 

'07 

Debaisieux 

1909 

Montgomery 

1911 

Kornhauser 

1914, 

'15 

Wenrich 

1916, 

'17 

Fasten 

1914, 

'18 

M  alone 

1918 

Pratt  and  Long 

1917 

Robertson 

1916 

Rabbit,  Man 

Tunicates,  Selachians,  Teleosts,  Am  pin- 
ox  us 

Tomopieris,  OphryotrochOf  Zodgonui, 
Enteroxeno8t  Myxine,  Salamandra^ 
Spinax 

( 'yclojM 

Thy8anozoon 

ZoogoniiA 

Batracoseps 

Alytes 

Planaria 

Dytiscus 

Euschislus 

Hersilia,  Enchenopa 

Phrynotettix,  Chorthipptu 

Cambarus,  Cancer 

Canis 

Mus 

Insects 


RUT 


HlTCROTIfll      niTOSIJ 


if-UKI)    CONTRACTION 


Honorific    riiioiti 


;u»  sriun 


».»«l«tJli 


Fig.  89. — The  method  of  chromosome  reduction  according  B. 

Explanation  in  text. 

Scheme  B. — The  second  of  the  two  conspicuous  interpretations  was 
advanced  by  Fanner  and   Moore   (1903,    L905),   and   i  ntially   as 

follows  (Figs.  89-92):     In  the  early  heterotypic  prophase  the  reticulum 
becomes   more   thready   in  structure  and  contracts  into  a   tight   knol 

(synizesis).     When   this    knot    loosens    up    tin'    chromatic    material    has 


238 


INTRODUCTION  TO  CYTOLOGY 


assumed  the  form  of  a  continuous  spireme  which  is  double.  This  double- 
ness  is  believed  to  represent  a  true  longitudinal  split,  and  although  it 
usually  disappears  from  view  during  the  later  prophases  it  is  thought  to 


Fig.   90. — The  heterotypic  prophases  in  Lilium,   according  to   Mottier   (1907.) 

A,  synizesis  knot  loosening  up;  threads  splitting;  note  chromomeres.     B,  hollow  spireme. 
C,  second  contraction.     D,  diakinesis.      X  900. 


■ 

2) 


m 


,  ■ 


-  ' 


-s, 


Fig.    91. — Maturation    mitoses    in    microsporocyte    of    Vicia  faba. 

A,  anaphase  of  heterotypic  mitosis;  split  for  second  mitosis  evident  in  separating 
daughter  chromosomes.  B,  one  daughter  nucleus  in  early  telophase  of  heterotypic  mitosis. 
C,  later  telophase.  D,  metaphase  of  homceotypic  mitosis.  E,  anaphase  of  same,  showing 
portions  of  both  spindles.  F,  three  of  the  four  microspore  nuclei.  X  1335.  (After 
Fraser,  1914.) 

persist  and  reappear  at  a  much  later  stage.  After  extending  loosely 
throughout  the  nucleus  ("open  spireme"),  the  double  spireme,  now  con- 
siderably thickened  and  twisted  (strepsinema) ,  contracts  again  and  is 


THE  REDUCTION  OF  THE  CHROMOSOMES 


thrown  into  loops  ("second  contraction").  These  loops  then  break 
apart  from  one  another  through  a  segmentation  of  the  spireme;  each  of 
them  is  composed  of  two  split  chromosomes  arranged  end-to-end.  ( Chro- 
mosome conjugation  has  thus  occurred  telosynapticaUy  {metasyndetic- 
ally)  either  while  the  spireme  was  being  formed  or  when  the  daughl 
spiremes  were  formed  in  the  preceding  telophase.  The  two  members  of 
each  pair  are  brought  around  to  a  side-by-side  position  by  the  Looping 
at  the  second  contraction,  usually  but  not  always  remaining  closely 
connected  at  the  original  point  of  conjugation.  The  resulting  bivalenl 
chromosomes,  with  their  split  obscured,  become  much  shortened  and 
thickened  (diakinesis)  and  take  up  their  positions  on  the  firsl  maturation 
spindle.  In  case  the  original  split,  instead  of  being  wholly  obscured,  in- 
visible at  this  time  or  earlier,  chromosome  tetrads  are  evident.  In  the 
heterotypic  anaphase  the  bivalents  are  separated  into  their  component 
univalents,  bringing  about  reduction.  During  the  anaphase  the  uni- 
valents often  widen  out  along  the  line  of  fission  which  had  been  tempo- 
rarily obscured,  giving  them  the  form  of  simple  or  double  Vs  as  described 
for  Scheme  A.  They  remain  through  interkinesis  in  the  double  condi- 
tion, and  in  the  homceotypic  mitosis  separate  along  this  line  of  fission. 

The  following  is  a  list  of  the  principal  works  in  which  this  theory  of 
eduction  has  been  advocated. 


Farmer  and  Moore 
Farmer  and  Digby 
Farmer  and  Shove 
Mottier 

Gregory 

Lewis 

Schaffner 

Digby 

Fraser 

Lawson 

McAvoy 

Beer 

Woolery 

Nothnagel 

Farmer  and  Moore 
Montgomery 

Moore  and  Embleton 

Griggs 

Zweiger 

H.  S.  Davis 

Nakahara 


Plants 

1903,  '05  Lilium,  Osmumla,  PxUotum,  Aneura 

1910  Galtonia 

1905  Tradescantia 

1907,    '09,    '14  Lilium,    Acer,    Allium,    Podophyllum, 

Tradescantia,  Staphylea 

1904  Ferns 

1908  Pinus,  Thuja 

1906,  '09  Agave 

1910,  '12,  '14,  '19  Galtonia,  Primula^  Crepis  Osmunda 

1914  Vicia 
1912  Smilacina 
1912  Fuchsia 

1912,  '13  Equisetum,  CrepiSy  Tragopogon 

1915  Smilacina 

1916  Allium 

Animals 

1905  Periplanela,  Elasmobrancha 
1903,  '04,  '05,  '06,  Hemiptera,  Amphibia 

'10 

1906  Amphibia 

1906  [scoria 

1907  Forficula 

1908  Insects 
1920  Perla 


240 


INTRODUCTION  TO  CYTOLOGY 


Some  of  the  above  named  investigators,  notably  Miss  Digby  (1910; 
1912,  1914,  1919),  Miss  Fraser  (1914),  and  Miss  Nothnagel  (1916), 
have  laid  emphasis  upon  the  view  that  the  split  seen  in  the  early  hetero- 
typic prophase  has  its  origin  in  the  telophase  of  the  last  premeiotic  divi- 
sion, each  chromosome  persisting  through  the  intervening  resting  stage 
in  the  double  condition.  It  is  consequently  held,  as  fully  stated  by  Miss 
Digby  (1919)  in  her  account  of  the  archesporial  and  meiotic  phases  of 
Os?nunda  (see  Fig.  92),  that  the  lateral  pairing  of  thin  threads  in  the 


LAST     PRCMtlOTIC     MITOSIS 


HETEROTYPIC     MITOSIS    : 


A 

ANAPHASE 


SYHIZE.SIS  "HOLLOW    SPiTtEM" 


COIOUC'ATION 


SE.COM3      CONTRACTION 


0 

V 


IlMillS.'- 


ANftTHftStS 


TELOPHASE 

(«ew  sflit) 


LATE 
TELOTHAS 


HonoTVPic  niTosis 

(70 


©. 0  ^>  <  >  d)  $r£ 


MtT«THAS£ 


ftWflPHflSE 


TtLOTKflSE 


Fig.  92. — Diagram  showing  behavior  of  chromosomes  in  premeiotic  and 
meiotic  phases  in  Osmunda,  according  to  Digby  (1919). 
a,  split  which  originates  in  telophase  of  premeiotic  mitosis,  persists  (though  obscured  at 
times)  through  heterotypic  prophases,  reappears  in  heterotypic  anaphase,  and  becomes 
effective  in  homceotypic  mitosis.  6,  split  which  originates  in  heterotypic  telophase, 
persists  obscured  through  homceotypic  prophases,  reappears  in  homceotypic  anaphase, 
and  becomes  effective  in  post-homceotypic  division,     x,  plane  of  conjugation. 

heterotypic  prophase  which  the  advocates  of  Scheme  A  have  regarded 
as  a  conjugation  of  entire  chromosomes  is  in  reality  only  the  reassocia- 
tion  of  the  two  halves  of  one  chromosome  which  had  been  split  in  the 
preceding  telophase.  Such  a  reassociation  is  thought  to  occur  in  every 
prophase,  somatic  and  heterotypic,  since  these  workers  regard  chromo- 
some splitting  as  regularly  a  thlophasic  phenomenon.  The  split  which 
forms  in  the  last  premeiotic  telophase  functions  in  the  homceotypic 
mitosis :  the  homceotypic  division  is  therefore  looked  upon  as  the  continua- 
tion of  the  premeiotic  division,  the  heterotypic  mitosis  being  an  inter- 
polated process  bringing  about  numerical  reduction.  Not  only  does  this 
premeiotic  split  reappear  in  the  anaphase  of  the  heterotypic  mitosis  to 
function  in  the  homceotypic,  but  a  new  split  develops  in  the  heterotypic 


THE  REDUCTION  OF  THE  CHROMOSOMES 


24] 


telophase,  and  after  being  temporarily  obscured  function-  in  the  post- 
homceotypic  division.1 

A  variation  of  Scheme  B  has  been  observed  in  (Enothera  I  rates  L908, 
1909,  1911;  Geerts  1908;  B.  M.  Davis  1909,  1910,  L911);  in  Fucus  and 
Cutleria  (Yamanouchi  1909,  1912);  in  Bufo  (King  1907;;  and  in  a  few 
other  forms.  Here  the  spireme  in  the  heterotypic  prophase  does  not 
become  double,  the  split  for  the  second  division  appearing  first  in  the 
heterotypic  anaphase. 

Comparison  of  Schemes  A  and  B. — According  to  both  of  the  fore- 
going prominent  theories  of  reduction  the  conjugated  chromosomes 
separate  at  the  first  maturation  mitosis,  thus  causing  reduction,  an. I 


Fig.  93. — Diagram  showing  distinction  between  Schemes  A  and  H.     See  text. 

divide  longitudinally  (equationally)  at  the  second  mitosis,  so  thai   the 
final  result  is  essentially  the  same:  two  of  the  resulting  four  nuclei  differ 
qualitatively  from  the  other  two  in  their  chromatin  content   (Fig.  '.' 
The  distinction  between  the  two  interpretations  is  nevertheless  an  im- 
portant one,  and  may  be  emphasized  in  the  following  summary. 

According  to  Scheme  A  the  double  character  of  the  chromatin  spiremes 
of  the  early  heterotypic  prophase  is  due  to  a  lateral  pairing  of  Bimple 
threads  each  representing  an  entire  somatic  chromosome,  the  second  con- 
traction not  being  significant  as  regards  pairing.  The  bivalent  chromo- 
somes so  formed,  after  much  shortening  and  thickening,  are  separated  in 
•the  heterotypic  mitosis,  during  the  anaphase  of  which  (or  earlier  in  the 

1  A  more  detailed  summary  of  this  view  may  be  found  in  §  review  <>f  Miaa  Digl 

paper  on  Osmunda  by  the  present  author  (1920(/ 
16 


242  INTRODUCTION  TO  CYTOLOGY 

case  of  chromosome  tetrads)  the  split  that  is  to  function  in  the  homceotypic 
mitosis  makes  its  appearance.  The  doubleness  in  the  heterotypic  pro- 
phase is  therefore  not  homologous  with  that  in  the  somatic  prophase: 
in  the  former  it  is  due  to  a  conjugation  and  in  the  latter  to  a  split. 

According  to  Scheme  B  the  doubleness  of  the  heterotypic  prophase  is 
due  to  a  true  splitting  as  in  the  case  of  somatic  division.  In  both  cases, 
moreover,  the  split  may  have  its  origin  in  the  preceding  telophase.  The 
bivalent  chromosome  is  formed  by  the  association  in  pairs  (often  at  first 
end-to-end  in  the  spireme  but  later  side-by-side)  of  segments  of  this  split 
spireme  at  the  time  of  the  second  contraction.  The  two  split  univalents 
composing  the  bivalent  are  separated  in  the  heterotypic  mitosis,  while 
in  the  homceotypic  mitosis  the  separation  is  along  the  line  of  the  split 
originating  in  the  last  premeiotic  telophase  and  seen  in  the  spireme  of  the 
early  heterotypic  prophase.  The  doubleness  of  the  early  heterotypic 
prophase  is  therefore  regarded  as  homologous  with  that  of  the  somatic 
prophase:  in  both  cases  it  represents  a  true  split. 

It  cannot  yet  be  said  what  the  outcome  of  this  controversy  is  to  be. 
The  advocates  of  Scheme  A  believe  that  those  of  Scheme  B  have  mis- 
interpreted the  changes  occurring  in  the  early  heterotypic  prophase  and 
in  all  telophases,  while  the  latter  charge  the  former  with  a  neglect  of  the 
second  contraction  stage.  Scheme  B  as  fully  elaborated  by  Miss  Digby 
has  certain  advantages:  it  allows  one  interpretation  to  be  placed  upon 
the  double  spireme  in  both  somatic  and  heterotypic  prophases,  irrespective 
of  the  exact  time  at  which  the  split  originates,  and  it  also  helps  to  explain 
the  sudden  appearance  of  the  split  for  the  second  maturation  mitosis  in 
the  anaphase  of  the  first.  Scheme  A,  on  the  other  hand,  is  preferred  by 
geneticists  because  of  the  earlier  and  much  longer  continued  association 
of  the  conjugating  chromosomes,  which  allows  a  greater  opportunity  for 
" crossing-over"  to  occur.  The  significance  of  this  point  will  be  brought 
out  in  Chapter  XVII. 

This  question,  however,  must  be  settled  primarily  by  direct  evidence. 
It  is  obvious  that  its  solution  depends  upon  the  exact  manner  in  which  the 
telophasic  transformation  of  the  chromosomes  and  the  derivation  of  the 
latter  from  the  reticulum  in  the  prophase  are  accomplished.  It  is  granted 
by  both  schools  that  the  alveolar  or  reticulate  condition  in  which  the 
chromosomes  are  found  in  late  telophase  is  continuous  with  the  similar 
condition  seen  in  the  succeeding  prophase.  If,  then,  it  is  true  (1)  that 
the  telophasic  transformation  (alveolation)  represents  a  true  splitting, 
and  (2)  that  the  early  prophasic  reticulate  condition  passes  directly 
into  the  double  spireme,  it  follows  that  this  doubleness  in  every  prophase 
is  due  to  the  split  originating  in  the  preceding  telophase.  But  workers  on 
mitosis  are  not  at  all  agreed  that  the  evolution  of  the  chromosomes  is 
that  stated  in  (1)  and  (2).  It  has  been  shown  in  Viciafaba  (Sharp  1913), 
Tradescantia  (Sharp  19206),  and  a  number  of  other  instances  (see  Chapter 


THE  REDUCTION  OF  THE  CHROMOSOMES  243 

VIII)  not  only  that  the  telophasic  alveolation  is  too  irregular  to  be 
regarded  as  a  splitting,  bu1  also  thai  the  reticulate  condition  of  the  pro- 
phase, instead  of  developing  directly  into  the  definitive  split,  gives  rise 

to  simple  thin  threads  in  which  a  new  split  is  developed.  From  this  it 
cannot  be  concluded  that  in  no  form  docs  the  split  develop  directly  from 
the  early  reticulate  condition,  or  thai  the  telophasic  alveolation,  though 
irregular,  may  not  later  become  so  equalized  as  to  constitute  the  firsl 
stages  of  the  split;  but  it  does  follow  thai  it  is  quite  unsafe  to  use  the 
principle  of  telophasic  splitting  as  a  premise  from  which  to  draw  tin- 
conclusion  that  the  approximation  of  thin  threads  in  the  early  heterotypic 
prophase  represents  the  reassociation  of  the  halves  of  a  single  split 
chromosome.  It  is  well  to  emphasize  the  possible  importance  of  the 
premeiotic  telophase,  but  any  ultimate  solution  of  this  perplexing  prob- 
lem must  be  reached  mainly  through  a  more  refined  analysis  of  th< 


Fig.  94. — Chromosome  pair  "B"  in  Phrynotcttix  magnus,  showing  condensation  of 
bivalent  pair  during  the  heterotypic  prophases  to  form  the  compact  chromosomes  appearing 
on  the  spindle  at  metaphase.      >!  1734.      (After  Wenrich,  1!»16.) 

prophasic  changes  which  have  led  a  long  list  of  investigators  to  the  con- 
clusion that  the  early  heterotypic  association  of  slender  threads  represents 
a  conjugation  of  entire  chromosomes  which  separate  in  the  firsl  matura- 
tion mitosis. 

One  of  the  most  convincing  pieces  of  direct  evidence  favoring  Scheme 
A  is  found  in  Wenrich's  recent  work  on  Phrynotettix  (1916).  Wenrich  is 
able  to  trace  a  single  pair  of  chromosomes,  distinguishable  by  their 
peculiar  form  and  the  arrangement  of  their  chromatic  accumulations  or 
chromomeres,  through  every  stage  from  the  spermatogonia  to  the  sperma- 
tids: During  the  heterotypic  prophase  the  two  members  of  the  pair 
conjugate  parasynaptically  while  in  the  form  of  slender  filaments.  Simi- 
larly strong  arguments  are  advanced  by  Robertson  L916)  as  the  result 
of  his  detailed  analysis  of  the  chromosome  groups  in  other  Tettigidffi 
and  Acrididae,  in  which  the  homologous  members  can  be  followed  with 
much  certainty  because  of  their  frequenl  inequality  in  size. 

Reduction  With  Chromosome  Tetrads.  As  already  pointed  out,  the 
marking  out  of  the  lines  of  separation  for  both  maturation  divisions 
during  the  heterotypic  prophase,  with  the  resulting  formation  of  chromo- 
some tetrads,  increases  in  no  inconsiderable  manner  the  difficulty  of 
interpreting  the  essential  changes  at  these  stag*  The  four  chromatids 
composing  the  tetrad  represent  two  conjugate. 1  chromosomes  each  of 
which  is  longitudinally  split.     Because  of  the  variety  of  ways  in  which 


244 


INTRODUCTION  TO  CYTOLOGY 


these  may  arrange  themselves  with  reference  to  one  another — in  the  form 
of  simple  or  compound  rods,  crosses,  and  rings — their  distribution  to  the 
daughter  nuclei,  as  well  as  the  manner  of  their  origin,  is  very  difficult  to 
follow  with  certainty.  The  accompanying  diagrams  will  serve  to  illus- 
trate the  more  common  modes  of  behavior  described  for  chromosome 
tetrads,  which  are  found  chiefly  in  the  cells  of  animals. 

Figure  95,  D  represents  an  exceptional  method  of  tetrad  formation 
described  by  Henking  (1891)  for  Pyrrochoris  and  by  Korschelt  (1895)  for 
Ophryotrocha.  The  continuous  spireme  segments  to  form  the  diploid 
number  of  chromosomes,1  which  then  split  longitudinally  and   shorten. 


D 


E 


>J> 


S" 


Fig.     95. — Reduction     with     chromosome     tetrads. 

D,  in  Pyrrochoris  (Henking)  and  Ophryotrocha  (Korschelt.)  E,  in  certain  copepods 
(Riickert,  Haecker,  and  vom  Rath.)  F,  in  Anasa  and  Allolobophora  (Paulmier;  Foot  and 
Strobell). 


No  conjugation  occurs  until  the  metaphase,  when  the  split  chromosomes 
come  together  end-to-end,  forming  tetrads.  They  at  once  separate  in 
the  anaphase,  bringing  about  reduction.  In  the  second  mitosis  they 
divide  along  the  original  split,  so  that  each  of  the  four  resulting  nuclei 
receives  the  haploid  number  of  chromosomes,  two  of  the  nuclei  thus 
differing  from  the  other  two  as  the  result  of  the  separation  of  entire 
(though  secondarily  split)  chromosomes  at  the  first  mitosis.  According 
to  Goldschmidt  (1905),  the  chromosomes  of  Zoogonus  minis,  after  thus 
undergoing  no  prophasic  conjugation,  divide  longitudinally  at  the  first 

1  For  the  sake  of  uniformity  and  clearness  the  diploid  number  is  represented  as  6 
in  all  of  these  diagrams. 


THE  REDUCTION  OF  Till-:  CHROMOSOMES  245 

mitosis  and  separate  into  two  haploid  groups  at   the  second.     To  this 

simple  form  of  reduction  Goldschmidl  applied  the  term  "  Primsertypus." 

Gregoire  (1909a),  on  the  contrary,  found  parasynapsis  and   the  usual 
mode  of  reduction  in  Zodgonus. 

The  interpretation  at  one  time  given  by  Ruckeri  (1893,  1894 
Haecker  (1895),  and  vom  Rath  (1895)  for  certain  copepoda  is  shown  in 
Fig.  95,  E.  The  continuous  spireme  splits  throughoul  its  length  and  then 
breaks  into  the  haploid  number  of  segments.  These  again  break  trans- 
versely, forming  chromosome  tetrads,  each  composed  of  two  split  chromo- 
somes arranged  end-to-end.  In  some  species  the  chromatids  open  oul  to 
form  four-parted  rings,  whereas  in  others  they  maintain  the  rod  form. 
A  separation  occurs  along  the  line  of  the  original  split  at  the  first  mitosis, 
which  is  therefore  equational,  and  along  the  plane  of  conjugation  ai  the 
second  mitosis,  which  is  therefore  reductions!.  In  Dicroccelium  Gold- 
schmidt  (1908)  reported  that  such  tetrads  divide  reductionally  at  the 
first  mitosis.  Lerat  (1905),  moreover,  has  found  that  in  Cyclops  strenu 
one  of  the  forms  used  by  the  earlier  workers,  the  tetrads  arise  by  a  parallel 
conjugation  of  thin  threads  which  later  split. 

A  third  mode  of  tetrad  behavior  is  that  reported  by  Paulmier  I  1899 
for  Anasa  tristis  and  by  Foot  and  Strobell  (1905,  1907)  for  Anasa  and 
Allolobophora  foetida  (Fig.  95,  F).  Here  the  chromosomes  conjugate 
end-to-end,  the  bivalents  so  formed  then  splitting  longitudinally,  giving 
tetrads  which  take  on  a  cross  or  ring  form.  At  the  first  mitosis  the  sepa- 
ration is  along  the  plane  of  conjugation,  effecting  reduction,  and  at  the 
second  it  is  along  the  plane  of  splitting.  According  to  McClung  1 1902 
Sutton  (1902,  1905),  Robertson  (1908),  and  others,  such  tetrads  separate 
reductionally  at  the  second  mitosis  (postreduction)  rather  than  af  the 
first  (prereduction)  in  certain  orthopterans  studied  by  them. 

Figure  96  illustrates  the  origin  of  chromosome  tetrads  of  five  charac- 
teristic types  by  the  two  prominent  modes  of  reduction  described  in 
detail  in  foregoing  pages.  According  to  Scheme  A  iJ,  Ci),  two  chromo- 
somes conjugate  parasynaptically  while  in  the  form  of  slender  threads. 
Instead  of  remaining  unsplit  as  in  most  plants,  each  member  then  -pin- 
longitudinally  in  a  plane  at  right  angles  to  the  conjugation  plane,  thus 
giving  a  tetrad  composed  of  four  parallel  strands  (chromatids  l>  . 
According  to  Scheme  B  (A2-C2),  the  two  chromosomes  are  at  firsl  ar- 
ranged telosynaptically  in  the  spireme  and  t  he  lat  ter  splits  t  hroughoul  it- 
length.  The  two  conjugating  members  then  take  up  a  side-by-side 
position,  and  their  split,  instead  of  becoming  obscured  as  usually  occurs 
in  plants,  remains  open,  giving  the  tetrad  of  parallel  strand-    D 

The  tetrad,  by  whichever  met  hod  it  has  arisen,  may  now  undergo  a 
variety  of  alterations,  some  of  which  are  shown  at  /•.'  and  /'.  Tin-  chroma- 
tids may  simply  shorten  and  t  hicken,  t  he  tel  rad  at  diakinesis  maintaining 
the  form  of  parallel  rods  (Ei,Fi).     They  may  open  out  along  the  plane  of 


246. 


INTRODUCTION  TO  CYTOLOGY 


conjugation  (E2)  and  take  the  form  of  rod  tetrads  (F2)  like  those  described 
by  Ruckert  and  Haecker.     While  opening  out  in  this  manner  the  longi- 


Fig.  96.— Diagram  showing  the  origin  of  the  tetrad  of  chromatids  (D)  according  to 
Scheme  A  (Ai-Ci)  and  Scheme  B  (A2-C2),  and  the  further  transformation  of  this  tetrad  into 
tetrads  of  five  types  (Fi-Fb) . 

tudinal  halves  of  each  chromosome  may  diverge  where  the  two  chromo- 
somes remain  in  contact  (#3),  the  tetrad  eventually  taking  the  form  of  a 
cross  (F3)  as  in  the  cases  described  by  Paulmier  and  by  Foot  and  Strobell. 


THE  REDUCTION  OF  THE  CHR0M0S0M1 


247 


If  the  conjugated  chromosomes  remain  in  contacl  al  both  ends  /. :  a 
complete  ring  results  (F4).  In  certain  orthopterans  the  four  chromatids 
open  out  along  the  conjugation  plane  in  some  regions  and  along  the  plain' 
of  splitting  in  other  regions;  this  results  in  the  curious  compound  rii 
(Fig.  156)  found  in  the  cells  of  these  insects.  Finally,  the  chromatids 
may  open  out  from  one  end  along  the  conjugation  plane  and  from  the 
other  end  along  the  splitting  plane  (#5),  the  tetrad  then  assuming  1 1n- 
form of  a  ring  composed  of  four  parts  (F-t).  In  all  cases  the  tetrads  usu- 
ally condense  into  compact  quadruple  bodies  by  the  time  they  take  their 
places  on  the  spindle  of  the  heterotypic  mitosis. 


STKtPil«tM/« 


©Q 


Fig.  97. — Reduction  with  chromosome  tetrads  in  Fasdola  h,  patica,  according 
to  Schellenberg   (1911).     Explanation    in    text. 


The  four  chromatids  composing  the  completed  tetrad  are  in  most  <• 
exactly  similar  in  appearance,  so  that  it  is  a  matter  of  much  difficulty  to 
determine  along  which  plane  they  arc  separated  at  the  firsl  maturation 
mitosis.  According  to  the  two  theories  of  tetrad  origin  illustrated  in 
the  foregoing  diagram,  however,  the  chromatids  are  supposed  in  aim* 
all  cases  to  separate  along  the  plane  of  conjugation  at  tin-  first  mitosis, 
and  this  conclusion  is  supported  by  the  behavior  of  those  bivalent  chromo- 
somes which  are  not  divided  into  tetrads  of  chromatids. 

A  further  interpretation  of  reduction  involving  chromosome  tetrads 
has  been  given  by  Sch<>]l(M)l)er-  I L91  1 1  for  the  parasitic  flatwopm,  I  ola 
hepatica  (Fig.  97).  The  chromatin  in  the  heterotypic  prophase  takes  the 
form  of  a  long  slender  filament  which  splits  longitudinally  soon  after 
synizesis.     This  double  thread  then  segments  into  the  haploid  number 


248  INTRODUCTION  TO  CYTOLOGY 

pieces,  each  representing  two  chromosomes  end-to-end;  these  have  the 
form  of  loops  with  a  definite  orientation  ("first  boquet  stage")-  Each 
segments  again,  giving  the  diploid  number  of  split  chromosomes,  which 
again  assume  the  form  of  oriented  loops  (" second  boquet  stage").  The 
halves  twist  tightly  about  each  other,  shorten  to  form  the  double  bodies 
seen  at  diakinesis  in  the  diploid  rather  than  the  haploid  number,  and  then 
conjugate  to  form  the  haploid  number  of  chromosome  tetrads.  The 
conjugating  members  (each  split)  separate  at  the  first  mitosis,  bringing 
about  reduction;  at  the  second  mitosis  the  separation  is  along  the  line  of 
the  original  split.  According  to  this  interpretation,  therefore,  the  double- 
ness  of  the  early  heterotypic  prophase  is  due  to  a  split,  as  in  Scheme  B, 
but  the  chromosomes  arranged  end-to-end  in  the  spireme  soon  become 
separated  and  do  not  conjugate  again  until  diakinesis. 

For  a  number  of  years  it  was  thought  (Carnoy  1886;  Boveri  1887; 
Hertwig  1890;  Brauer  1893)  that  the  chromosome  tetrad  in  Ascaris 
megalocephala  was  exceptional  in  being  formed  by  two  longitudinal  fis- 
sions of  a  primary  chromatin  rod,  there  being  as  a  consequence  no  quali- 
tative reduction  in  the  two  maturation  divisions  unless  the  organization 
of  the  chromatin  were  different  from  that  of  other  organisms.  But  it  has 
since  been  shown  that  they  arise  as  in  other  organisms  by  the  conjugation 
of  two  split  chromosomes  (Sabaschnikoff  1897;  Tretjakoff  1904;  Griggs 
1906).  In  the  oogenesis  Griggs  reports  telosynapsis  with  prereduction, 
whereas  in  the  spermatogenesis  Tretjakoff  describes  parasynapsis  followed 
by  postreduction.  In  Ascaris  canis  (Marcus  1908;  Walton  1918)  the 
four  chromatids  each  show  a  transverse  constriction,  the  chromosomes  on 
the  first  maturation  spindle  having  the  form  of  octads. 

Although  the  formation  of  well  differentiated  chromosome  tetrads 
occurs  very  commonly  in  animals,  it  appears  to  be  very  rare  in  plants. 
Farmer  (1895)  described  tetrads  in  Fossombronia,  and  they  have  since 
been  reported  in  at  least  three  other  bryophytes:  Pallavicinia  (Moore 
1905),  Sphagnum  (Melin  1915),  and  Chiloscyphus  (Florin  1918).  They 
have  also  been  described  in  a  few  vascular  plants :  Equisetum  (Osterhout 
1897),  Pteris  (Calkins  1897),  Ariscema  (Atkinson  1899),  Tricyrtis  (Ikeda 
1902),  Thalictrum,  Calycanthus,  and  Richardia  (Overton  1909)  (Fig.  98), 
Spinacia  (Stomps  1911),  Primula  (Digby  1912),  and  Lopezia  (Tackholm 
1914). 

According  to  Gregoire  (1905)  such  structures  in  plants  are  not  true 
tetrads,  but  resemble  them  because  the  chromosomes  are  often  bent  and 
have  their  material  accumulated  largely  at  their  ends.  Sakamura 
(1920)  interprets  them  as  conjugated  constricted  chromosomes,  and  denies 
that  the  quadripartite  condition  has  anything  to  do  with  reduction  in 
such  cases.  He  likewise  accounts  for  the  metasynaptic  rod  tetrads  (Fig. 
95,  E)  described  by  several  investigators  of  maturation  in  animals, 
holding  that  they  represent  two  constricted  chromosomes    conjugated 


THE  REDUCTION  OF  THE  CHROMOSOMES 


249 


parasynaptically  rather  than  two  split,  ones  placed  end-to-end.  In 
support  of  this  contention  he  cites  the  following  observations:  Buch 
"tetrads"  are  seen  not  only  in  the  oocytes  and  spermatocytes  bul  also  in 
oogonia,  spermatogonia,  and  somatic  cells;  the  supposed  telosynaptically 
conjugated  members  are  often  very  unequal  in  size;  such  tetrads  are 
sometimes  divided  in  the  transverse  plane  at  neither  maturation  mitosis  ; 
not  only  tetrads,  but  also  octads  and  hexads  are  often  observed,  even  in 


#  i 


at  * 

A 


Fig.  98. — Chromosome  tetrads. 

A,  five  stages  in  the  development  of  the  tetrad  in  the  spermatocyte  of 
X  3000.     From  a  preparation  by  Dr.  H.  E.  Stork.     B,  tetrads  in  sporocyte  of  Chii 
Enlarged;     X  2800.     (After   Florin,     1918.)      C,    tetrads    in    Richardia    afrieana. 
Overton,  1909.)     D,  false  tetrads  in  somatic  cells  of  Pisum  due  to  action  ol  chloral  hydi 

on  constricted  chromosomes.      (After  Sakamura,  1920.) 

the  same  cell,  and  these  are  plainly  due  to  the  presence  of  additional 
accentuated  constrictions.  Robertson  (1916)  also  interprets  such  telo- 
synaptic  rod  tetrads  as  those  observed  by  Baecker  in  the  copepods 
constricted  chromosomes.  The  constrictions,  according  to  this  writer, 
represent  points  of  temporary  union  between  non-homologoua  element 
From  these  considerations  it  is  evident  thai  constrictions  have  much  to 
do  with  the  appearances  assumed  by  chromosomes,  and  that  they  should 
be  taken  into  account  in  interpreting  the  chromosome  tetrad. 

Numerical  Reduction  Without  Qualitative  Reduction.— Figure  99 
illustrates  the  behavior  of  the  chromosomes  in  maturation  according  to 
three  not  widely  accepted  views.     A  few  workers,  including  Kick  |  L907, 


250 


INTRODUCTION  TO  CYTOLOGY 


1908),  Meves  (1907,  1908,  1911),  Giglio-Tos  (1908),  and  Granata  (1910), 
reject  the  theor}-  of  chromosome  individuality  and  specificity,  and 
therefore  do  not  regard  the  chromosomes  which  are  distributed  to  the 
four  cells  at  maturation  as  at  all  identical  with  those  of  the  divisions  im- 
mediately preceding,  except  in  so  far  as  they  are  composed  of  the  same 
nuclear  material.  Accordingly  they  recognize  no  qualitative  reduction, 
but  only  a  numerical  one.  This  reduction  in  number  results  from  the 
fact  that  the  spireme  formed  in  the  heterotypic  prophase  (Fig.  99,  A) 
segments  into  the  haploid  number  of  pieces  instead  of  the  diploid  number, 
these  pieces  being  simply  divided  longitudinally  at  both  maturation 
divisions,  and  the  four  resulting  nuclei  being  qualitatively  similar. 


.i 


fin  (0)  (? 


B 


C 


kA  © 


Fig.  99. — Diagram  showing  three  reported  modes  of  numerical  reduction  with- 
out   qualitative    reduction. 
A,  according  to  Fick  et  al.     B,  according  to  Vejdowsky;  complete  fusion  of  conjugating 
members.     C,    according   to    Bonnevie;    bivalents   arranged  on  spindles  in  juxtaposition; 
fusion  of  conjugating  members  eventually  becomes  complete. 

According  to  Vejdowsky  (1907)  (Fig.  99,  B)  the  chromosomes  appear 
in  diploid  number  in  the  heterotypic  prophase  and  conjugate  parasynapti- 
cally.  The  members  of  the  pair  fuse  completely  and  lose  their  individual 
identity,  so  that  the  chromosomes  appearing  on  the  first  maturation 
spindle  in  haploid  number  are  new  entities,  and  not  merely  temporary 
pairs  of  somatic  chromosome  individuals.  At  both  divisions  these  bodies 
split  longitudinally,  giving  equivalence  to  the  four  resulting  nuclei. 
Here,  as  in  the  foregoing  example,  there  is  no  definite  qualitative  reduc- 
tion in  Weismann's  sense,  though  a  numerical  reduction  is  brought  about 
by  means  of  a  complete  fusion  at  the  time  of  chromosome  conjugation. 


THE  REDUCTION  OF  THE  CHR0M0S0M1  25] 

An  interpretation  put  forward  by  Bonnevie  (1906,  L908)  is  shown  In 
Fig.  99,  C.  Here  the  chromosomes  conjugate  parasynaptically  and 
come  into  very  intimate  union:  although  fchey  appear  to  undergo  a  real 
fusion  their  identity  is  maintained  for  a  time.  Owing  to  the  fact  thai 
these  bivalent  chromosomes  are  inserted  upon  the  Bpindle  with  their 
halves  in  juxtaposition  (side-by-side  with  respect  to  the  poles)  rather  than 
in  superposition  (one  toward  each  pole),  the  members  of  a  conjugated 
pair  separate  neither  at  the  first  division  nor  at  the  second.  As  a  result 
each  of  the  four  cells  receives  the  haploid  number  of  chromosomes,  all 
of  which  are  bivalent,  and  no  qualitative  reduction  occurs.  Bonnevie 
believes  that  the  conjugating  members  of  each  pair  finally  fuse  completely 
in  the  subsequent  stages.  In  this  case,  therefore,  as  in  the  preceding  one 
numerical  reduction  is  supposed  to  result  from  a  complete  fusion  of  the 
chromosomes  in  pairs. 

Whether  any  confidence  is  to  be  placed  in  such  interpretations  or  not 
— and  according  to  most  cytologists  none  should  be — they  at  least  serve 
to  show  how  it  is  possible  that  numerical  reduction  may  occur  without 
effecting  any  qualitative  reduction,  and  that  the  essential  feature  of  tin 
reduction  of  the  chromosomes  is  something  other  than  the  mere  change 
in  their  number,  as  pointed  out  at  the  beginning  of  the  chapter. 

SYNAPSIS,  OR  CHROMOSOME  CONJUGATION 

The  phenomenon  of  chromosome  conjugation,  or  synapsis,  which  we 
have  seen  above  is  such  an  important  feature  of  the  reduction  process, 
must  now  be  somewhat  more  closely  examined.  Attention  will  !><• 
directed  to  three  points:  the  relationship  of  the  conjugating  memfo 
(the  "synaptic  mates"),  the  stage  at  which  the  synaptic  union  taki  - 
place,  and  the  exact  nature  of  this  union. 

Relationship  of  the  Synaptic  Mates.— We  may  first  inquire  into  tin 
relationship  which  may  exist  between  the  two  chromosomes  pairing  to 
form  a  given  bivalent  chromosome:  is  any  chromosome  of  the  duplex 
group  (the  two  intermingled  parental  chromosome  sets  in  the  individual's 
nuclei)  present  in  the  gonotokont  free  to  pair  wit  h  any  other  chromosome, 
or  does  the  pairing  take  place  according  to  more  restricting  nil 

It  was  suggested  by  Henking  (1891)  that  the  two  synaptic  mat.-  are 
ultimately  derived  from  the  two  parents  at  the  previous  fertilization, 
one  from  the  father  and  the  other  from  the  mother:  the  chromosomes  of 
one  parental  set  pair  with  those  of  the  other  parental  set  to  form  the 
haploid  number  of  bivalent  chromosomes  appearing  on  the  fust  matura- 
tion spindle.  This  idea  was  later  emphasized  and  developed  by  Mont- 
gomery (1900-4),  Sutton  (1902),  Boveri  (1901),  and  others,  who  found 
for  it  much  supporting  evidence  in  organisms  with  chromosomes  differ- 
ing in  size  and  shape.  An  observation  made  by  Rosenberg  L909  on 
Drosera  hybrids  is  significant  in  this  connection.      When  Drosera  rotundi- 


252  INTRODUCTION  TO  CYTOLOGY 

folia  (20  chromosomes)  is  crossed  with  D.  longifolia  (40  chromosomes) 
there  results  a  hybrid  with  30  chromosomes,  of  which  10  are  contributed 
by  rotundifolia  and  20  by  longifolia.  When  synapsis  occurs  preparatory 
to  reduction  in  this  hybrid  only  10  bivalents  are  formed,  10  chromosomes 
remaining  unpaired.  This  was  taken  by  Rosenberg  to  mean  that  the  10 
rotundifolia  chromosomes  pair  with  10  of  the  longifolia  ones,  leaving  the 
other  10  of  longifolia  without  synaptic  mates.  Had  any  chromosome  of 
the  duplex  group  of  30  been  free  to  pair  with  any  other,  15  bivalents 
would  have  been  produced. 

Other  instances  of  this  phenomenon  may  be  mentioned.  By  crossing 
(Enothera  Lamar xkiana  (seven  chromosomes  in  gamete)  with  CE.  gigas  (14 
in  gamete)  individuals  with  21  chromosomes  are  obtained.  Geerts 
(1911)  found  that,  preparatory  to  reduction,  the  seven  Lamarckiana  chro- 
mosomes pair  with  seven  of  the  gigas  chromosomes,  leaving  the  other 
seven  of  gigas  unpaired.  On  the  contrary,  however,  Gates  (1909)  found 
that  the  21  chromosomes  in  a  lata-gigas  hybrid  simply  separate  into  two 
approximately  equal  groups,  usually  of  10  and  11  chromosomes  re- 
spectively. Kihara  (1919)  reports  that  in  some  35-chromosome  wheat 
hybrids  formed  by  crossing  Triticum  poionicum  (14  chromosomes  in 
gamete)  with  T.  spelta  (21  in  gamete)  there  are  present  in  the  heterotypic 
prophase  14  bivalents  (poionicum  conjugated  with  spelta)  and  seven 
univalents  (spelta).  The  14  bivalents  are  arranged  on  the  spindle  and 
separate  as  usual,  whereas  the  seven  unpaired  spelta  chromosomes  split 
longitudinally  at  the  first  mitosis  and  distribute  themselves  irregularly 
at  the  second  (Fig.  100).  An  analogous  condition  is  found  in  Pigcera 
hybrids  by  Federley  (1913). 

A  very  significant  additional  suggestion  with  respect  to  synapsis  was 
made  by  McClung  (1900)  and  Sutton  (1902) :  not  only  are  the  two  chromo- 
somes which  conjugate  derived  from  the  two  parents,  but  they  are  hom- 
ologous— each  chromosome  of  one  parental  set  pairs  with  a  particular 
chromosome  of  the  other  parental  set,  the  two  members  of  the  resulting 
bivalent  being  presumably  of  corresponding  hereditary  value,  as  will  be 
shown  in  Chapter  XV.  The  evidence  for  this  important  hypothesis  was 
found  chiefly  in  Brachystola  (Fig.  101)  and  a  number  of  other  insects 
having  chromosome  complements  made  up  of  members  with  constant 
characteristic  differences  in  size  and  shape.  Many  such  cases  have  been 
subsequently  discovered,  especially  by  McClung  and  his  coworkers  in 
their  extensive  researches  on  insect  spermatocytes.  As  examples  among 
plants  may  be  cited  Crepis  virens,  Najas  major,  N.  marina,  and  Vicia 
faba. 

Crepis  virens  (Rosenberg  1909)  (Fig.  102)  has  six  chromosomes:  two 
long,  two  medium  sized,  and  two  short.  When  synapsis  occurs  the  like 
chromosomes  pair,  forming  bivalents  of  three  sizes.  The  members  of 
each  pair  separate  and  pass  to  the  daughter  cells  at  the  first  maturation 


THE  REDUCTION  OF  THE  CHROMOSOMES 


253 


mitosis,  each  microspore  (after  the  second  mitosis)  having  as  a  result 
three  chromosomes:  one  long,  one  medium  sized,  and  one  short.  Since 
the  gamete  receives  such  a  simplex  group  of  three  chromosomes,  and  the 


4^ 


Fig.  100. — Heterotypic  mitosis  in  Triticum  polonicum  X  T.  *j»lta. 

A,  the  21  chromosomes  (polar  view).  B,  14  bivalents  separated  into  component 
univalents;  7  unpaired  spelta  chromosomes  have  split  and  are  about  to  be  distributed. 
(After  Kihara,  1919.) 

somatic  cells  of  the  new  individual  show  six  (two  of  each  length),  it  is 
evident  that  the  other  gamete  furnishes  a  similar  simplex  group  of  three. 


Fig.  101. — The  chromosome  complement  in  the   spermatocyte  of  B 

magna.     {After  Sutton,  L902. 

In  Najas  marina  and  Najas  major  (Muller  l'.tll;  Tschernoyarow 
1914)  the  duplex  group  of  14  chromosomes  is  made  up  of  seven  visibly 
different  pairs  (Fig.  56  bis).  In  the  heterotypic  prophase  these  conjugate 
selectively  to  form  seven  bivalents,  the  reduced  nuclei  therefore  receiving 


254 


INTRODUCTION  TO  CYTOLOGY 


a  set  of  seven  visibly  different  chromosomes.     Sakamura   (1920)  holds 
the  number  here  to  be  six  rather  than  seven.     (See  p.   160.) 

In  Vicia  faba  (Sharp  1914;  Sakamura  1915,  1920)  there  are  in  the 
somatic  cells  12  chromosomes,  two  of  them  being  about  twice  as  long  as 
the  other  10  (Figs.  56  and  102).  At  synapsis  in  the  microsporocyte  there 
are  formed  six  bivalents,  one  of  them  having  about  twice  the  length  of 
the  other  five.  Hence  it  is  clear  that  the  two  long  chromosomes  pair 
with  each  other.     In  the  heterotypic  mitosis  the  synaptic  mates  separate 


SPORES    - 

/  THOBuCt       r     MMETOFttlTtS 

An£     f   t*ntT£i      with 
3Anl     CHhonoiOMt      SET 


£0DT    CLLL 


SPOROCYTC 


VICIA      FABA 


SIMILAR    PROCESS      IN      P 


CREPIS     VIRLNS 


SIMILAR      PROCESS      I 


Fig.    102. — Chromosome   cycles  in    Vicia  faba  and   Crepis  virens,   showing 

homologous    pairing. 

and  pass  to  the  daughter  nuclei,  bringing  about  reduction.  At  the  close 
of  the  homceotypic  mitosis  the  microspore,  and  hence  the  male  gamete 
to  which  it  later  gives  rise,  receives  a  simplex  group  of  six  chromosomes: 
one  long  and  five  short.  Since  the  somatic  cells  contain  each  of  these  in 
duplicate  it  is  evident  that  a  similar  set  is  contributed  by  the  female 
gamete. 

Summing  up,  we  may  draw  from  the  above  facts  certain  very  impor- 
tant conclusions:  (1)  Each  parent  furnishes  the  offspring  with  a  set  of 
chromosomes,  the  members  of  the  two  sets  being  intermingled  in  all  the 
nuclei  of  the  new  individual.  This  point  will  receive  further  attention  in 
the  following  chapter  on  fertilization.  (2)  The  two  members  of  each 
bivalent  chromosome  formed  at  synapsis  are  derived  one  from  each  parental 
set.  (3)  Each  chromosome  of  the  paternal  set  conjugates  with  a  particular 
chromosome  of  the  maternal  set:  the  two  are  in  some  sense  homologous. 


THE  REDUCTION  OF  THE  CHROMOSOMES 

It  should  be  pointed  out  thai  cytologists  and  genet  Icisl  -  have  generally 
assumed  that  each  synaptic  pair  is  independent  of  all  the  others  as 
regards  the  manner  in  which  it  is  oriented  on  the  heterotypic  spindle. 
In  some  pairs  the  paternal  members  are  directed  toward  one  pole  and  in 
other  pairs  toward  the  other  pole.  It  is  conceivable  that  in  Bome 
all  the  paternal  members  might  go  to  one  pole  and  all  the  maternal 
members  to  the  other.  Direct  evidence  that  the  assortment  of  the 
various  chromosome  pairs  is  in  this  respeel  a  random  one  :i-  originally 
assumed  has  been  furnished  by  Miss  Carothers  (1913,  L917  .  In  the 
grasshopper,  Trimerotropis,  she  finds  that  the  components  of  some  of  the 
bivalents  are  visibly  different  in  size,  in  the  mode  of  attachment  to  the 
spindle  fibers,  and  in  the  presence  of  constrictions;  and  that  these  differ- 
ences make  it  possible  to  show  beyond  question  that  the  several  pairs  an- 
entirely  independent  of  one  another  as  regards  their  orientation  on  the 
spindle  and  their  consequent  distribution  to  the  daughter  cells. 

From  the  precise  manner  in  which  the  distribution  of  chromosomes 
at  the  time  of  reduction  and  at  other  stages  of  the  life  cycle  parallels  the 
distribution  of  the  hereditary  characters  it  is  inferred  that  Buch  hom- 
ologous chromosome  pairs  represent  the  material  basis  for  the  allelo- 
morphic  pairs  of  Mendelian  characters  exhibited  by  the  organism.  This 
subject  is  to  be  taken  up  in  Chapter  XV. 

The  Stage  at  Which  Conjugation  Occurs. — In  the  great  majority  of 
observed  cases  chromosome  conjugation  occurs  during  the  prophi 
of  the  first  maturation  division.  Since  the  chromatin  threads  at  some 
time  during  these  prophases  usually  take  the  form  of  a  tightly  contracted 
knot  out  of  which  they  emerge  in  an  obviously  double  condition,  it  was 
suggested  (Moore  1896)  that  the  contraction  is  an  important  factor  in 
bringing  about  the  conjugation,  and  the  contraction  itself  came  t<»  be 
called  "synapsis."  But  an  examination  of  the  various  modes  of  reduction 
shows  that  the  conjugation  may  begin  very  early,  before  the  contraction 
(Fig.  83)  or,  on  the  other  hand,  not  until  the  spindle  is  established 
(Fig'.  95,  D).  The  conjugation  of  the  chromosomes  is  therefor.-  to  be 
distinguished  from  the  contraction.  It  has  now  become  customary  to 
refer  to  the  former,  at  whatever  stage  it  occurs,  as  synapsis,  and  to  the 

latter  as  synizesis. 

In  an  increasing  number  of  reported  cases  the  paired  a  — .nation 
apparently  begins  even  before  the  heterotypic  prophase.  The  dim  mo- 
somes  have  been  observed  in  several  instances  to  undergo  pairing  during 
the  anaphase  and  telophase  of  the  hist  premeiotic  division.  Such  is  the 
condition  in  certain  Hemiptera  (Montgomery  L900,  1901  .  Onis* 
(Nichols  1902),  Brachystola  (Sutton  L902),  Scolopendra  (Blackman 
1903,  1905),  Pedicellina  (Dublin  1905),  and  a  number  of  more  recent 
cases.  Furthermore,  the  pairing  has  been  stated  to  begin  in  the  sperm- 
atogonia several  cell  generations  before  maturation  in  certain  Hemiptera 


256  INTRODUCTION  TO  CYTOLOGY 

and  Ascaris  (Montgomery  1904,  1905,  1908,  1910),  Alytes  (Janssens 
and  Willems  1909),  Helix  and  Sagitta  (Stevens  1903;  Ancel  1903),  certain 
Diptera  (Stevens  1908,  1911),  and  Pediculus  (Doncaster  1920). 

More  recently  it  has  been  shown  that  the  homologous  chromosomes 
may  begin  to  show  a  paired  arrangement  even  earlier  in  the  cycle,  in 
some  cases  directly  after  the  parental  groups  are  brought  together  at 
fertilization.  In  the  Diptera,  for  example,  Metz  (1916a)  has  shown  that 
the  association,  which  at  certain  stages  is  so  close  as  to  constitute  a 
synapsis,  begins  before  the  cleavage  of  the  fertilized  egg,  and  that  the 
paired  condition  is  maintained  in  all  cells,  somatic  and  germinal,  through- 
out the  life  cycle.  Metz  examined  80  species  and  in  all  of  them  found 
such  a  somatic  pairing.  In  Culex  (Stevens  1910,  1911;  Taylor  1914, 
1917),  which  has  six  chromosomes,  the  association  can  be  seen  in  the 
nuclei  of  the  segmenting  egg,  and  in  the  early  larval  stages  there  follows 
an  actual  parasynaptic  fusion,  so  that  the  somatic  cells  thereafter  show 
three  bivalent  chromosomes  rather  than  six  univalents.  In  the  matura- 
tion divisions  the  members  of  each  pair  separate,  the  gametes  receiving 
three  chromosomes  each,  just  as  they  would  had  conjugation  begun  in 
the  heterotypic  prophase  as  usual. 

A  loosely  paired  arrangement  of  the  chromosomes  in  the  somatic 
cells  of  plants  has  been  reported  by  Strasburger  (1905,  1907,  1910)  for 
Galtonia  candicans,  Funkia  Sieboldiana,  Pisum  sativum,  Melandrium, 
Mercurialis,  and  Cannabis;  by  Sykes  (1908)  for  Hydrocharis,  Lychnis, 
Begonia,  Funkia,  and  Pisum;  by  Overton  (1909)  for  Calycanthus;  by 
Muller  (1909,  1911)  for  Yucca  and  other  forms;  by  Stomps  (1910,  1911) 
for  Spinacia;  by  Kuwada  (1910)  for  Oryza;  by  Tahara  (1910)  for  Morus; 
and  by  Ishikawa  (1911)  for  Dahlia.  This  is  another  matter  that  will  be 
considered  further  in  Chapter  XII. 

The  Nature  of  the  Synaptic  Union. — Because  of  the  manner  in  which 
chromosome  behavior  is  at  present  being  applied  to  the  solution  of  the 
problems  of  inheritance,  no  question  concerning  chromosome  conjugation 
is  more  important  than  that  of  the  exact  nature  of  the  synaptic  union. 
In  reviewing  some  of  the  opinions  of  this  subject  it  will  be  convenient 
to  list  separately  the  views  of  the  telosynaptists  and  the  parasynaptists. 

In  such  cases  as  those  described  by  Henking  and  by  Goldschmidt 
Fig.  95,  D)  there  is  only  a  momentary  end-to-end  association  of  the  fully 
formed  chromosomes  on  the  spindle  of  the  heterotypic  mitosis,  there 
being  no  real  fusion  and  almost  no  opportunity  for  an  "  interchange  of 
influences."  In  the  other  tetrad  chromosomes  formed  by  telosynapsis 
(Fig.  95,  E  and  F;  Fig.  90)  there  is  only  slightly  greater  opportunity 
for  such  interchange.  According  to  Scheme  B  (Fig.  89)  the  synaptic 
mates  are  at  first  arranged  end-to-end,  and  only  later,  when  partially 
condensed,  do  they  take  up  a.  side-by-side  position,  allowing  a  more 
intimate  and  extensive  union  for  a  short  time. 


THE  REDUCTION  OF  Till-:  CHROMOSOMES  257 

Generally  speaking,  the  parasynaptists  have  given  more  attention 

to  the  details  of  the  synaptic  union  than  have  the  telosynaptisl  Al- 
though cases  are  on  record  in  which  there  is  only  a  momentary  para- 
synaptic  association  of  fully  formed  chromosomes  (von  Vosa  l'.»l  1 
the  association  usually  extends  over  considerable  time.  Most  para- 
synaptists hold  that  the  conjugation  begins  with  the  association  of  the 
leptotene  threads  before  or  during  synizesis,  continues  through  the 
remainder  of  the  prophase,  and  ends  with  the  anaphasic  separation 
(Scheme  A).  The  association  of  the  synaptic  mates  is  thus  long  and 
intimate.  Concerning  the  closeness  of  the  union,  however,  opinions 
differ  widely. 

A  few  investigators  (Vejdowsky  1907;  Bonnevie  1906,  1908,  1911; 
Winiwarter  and  Sainmont  1909;  Schneider  1914)  have  thoughi  that  tin- 
conjugating  members  fuse  completely  and  lose  their  individual  identity. 
the  "mixochromosome''  so  formed  then  undergoing  two  true  longitudinal 
splits  along  new  planes  at  the  twTo  maturation  divisions.  In  some  <•. 
(Bonnevie;  see  p.  251)  the  fusion  may  not  be  fully  consummated  until 
during  the  post-meiotic  divisions.  Others  believe  the  split  for  the 
heterotypic  mitosis  to  be  along  the  plane  of  conjugation  (Cardiff  1906, 
Fasten  1914,  and  others).  Probably  the  most  widely  advocated  view  is 
that  there  is  no  actual  fusion  of  the  synaptic  threads,  the  latter  main- 
taining their  identity  completely.  Although  their  association  may  at 
times  be  so  intimate  that  they  seem  to  constitute  a  single  thick  thread, 
the  doubleness,  if  thus  lost  to  view,  reappears  during  later  stages    I  terghs 

1904,  1905;  A.  and  K.  E.  Schreiner  1905,  1906;  Marechal  1907;  Overton 

1905,  1909;  Robertson  1915,  1916)  (Fig.  88).  Several  careful  observers 
have  reported  that  the  doubleness  can  be  seen  at  all  stages  (Gregoire 
1907,  1910;  Schleip  1906,  1907;  Montgomery  1911;  Kornhauser  191  1. 
1915;  Wenrich  1915,  1917).  Gregoire,  who  has  argued  strongly  for  this 
interpretation,  has  emphasized  the  ease  with  which  the  closely  appressed 
threads  may  be  mistaken  for  a  single  thick  structure. 

One  of  the  most  important  suggestions  which  has  been  made  concern- 
ing chromosome  conjugation  is  embodied  in  the  lt Chiasmatype  Hypo- 
thesis" of  Janssens  (1909).  According  to  Janssens,  the  pairing  threads, 
though  remaining  separate  throughout  the  greater  part  of  their  length, 
fuse  at  one  or  more  points  as  they  twist  about  each  other.  When  they 
again  separate  a  break  occurs  at  each  of  these  fusion  point-,  but  along  a 
new  plane,  so  that  each  of  the  two  resulting  chromosomes  is  composed 
of  portions  of  both  conjugating  members  (Fig.  1  19  This  interpreta- 
tion, which  has  been  admitted  as  possible  by  several  of  the  investigate 
named  in  the  preceding  paragraph,  is  significant  in  that  it  shows  how  an 
orderly  evolution  of  chromosomes  with  new  constitutions  may'occur,  a 
point  of  great  importance  in  connection  with  current  conceptions  of  tic 

17 


258  INTRODUCTION  TO  CYTOLOGY 

physical  basis  of  heredity.     Special  attention   will  be  devoted  to  this 
question  in  Chapter  XVII. 

Chromomeres.— An  important  role  has  been  attributed  to  the  chro- 
momeres  by  many  students  of  synapsis.  Allen  (1905),  for  example, 
maintained  that  the  fusion  of  the  leptotene  threads  in  Lilium  involves  a 
fusion  of  their  chromomeres,  the  subsequent  division  of  the  fused  chro- 
momeres initiating  the  resplitting  of  the  pachytene  thread.  Allen  found 
the  chromomeres  to  be  composed  of  still  smaller  chromatic  elements, 
and  offered  various  suggestions  concerning  the  manner  in  which  the  re- 
splitting  of  the  pachytene  thread  might  be  supposed  to  effect  a  redistri- 
bution of  the  "idioplasms."  That  chromosome  conjugation  is  primarily 
a  conjugation  of  small  chromatic  elements  within  the  chromosome  was 
held  by  Strasburger  (1904,  1905)  and  the  Schreiners  (1906).  The  visible 
chromatin  granules,  or  "pangenosomes,"  were  conceived  by  Strasburger 
to  represent  complexes  of  "pangens"  such  as  were  postulated  by  de  Vries, 
conjugation  involving  an  interchange  of  these  latter  units. 

The  chromomere  ^  interpretation  has  been  adversely  criticised  by 
Gregoire  (1907,  1910)  on  the  basis  of  further  evidence  obtained  from  a 
study  of  the  chromatic  structures  themselves.  This  author  points  out 
several  serious  objections  to  the  view  that  the  chromomeres  are  auton- 
omous bodies,  and  concludes  that  they  are  rather  to  be  regarded  simply 
as  swellings  or  thicker  portions  of  the  chromatin  thread,  such  thick  por- 
tions remaining  as  the  thread  undergoes  a  stretching  which  is  not  uni- 
formly resisted  at  all  points.  Their  frequently  striking  correspondence 
or  paired  arrangement  in  the  synaptic  threads  is  explained  as  the  result 
of  the  response  of  the  two  closely  associated  threads  to  the  same  stretch- 
ing force.  This  interpretation  is  also  shown  to  account  for  the  variability 
in  the  dimensions  of  the  chromomeres,  their  tapering  form,  the  often 
reported  absence  of  correspondence  between  the  chromomeres  of  the 
two  threads,  and  various  other  aspects.  Wenrich  (1916,  1917),  on  the 
other  hand,  has  found  that  in  Phrynotettix  (Fig.  155)  the  chromomeres 
show  a  remarkable  individual  constancy  in  size  and  position  in  a  given 
member  of  the  chromosome  complement,  not  only  in  the  various  cells  of 
a  given  individual,  but  also  in  those  of  different  individuals.  These 
facts  strongly  suggest  an  autonomy  of  the  bodies  in  question. 

Because  of  their  great  theoretical  importance  (see  Chapter  XVII)  it 
is  to  be  regretted  that  after  such  a  large  amount  of  research  so  many 
points  regarding  the  process  of  synapsis  should  remain  in  such  an  un- 
settled state.  It  is  hoped  that  further  refinements  in  microtechnique 
may  remove  some  of  the  obscurity  which  at  present  surrounds  them. 

OTHER   OPINIONS  ON   THE  HETEROTYPIC  PROPHASE 

Although  the  phenomena  of  the  heterotypic  prophase,  particularly 
synizesis  and  synapsis,  are  generally  looked  upon  as  normal  occurrences 


THE  REDUCTION  OF  THE  CHROMOSOMES 

of  considerable  significance,  not  all  investigators  concur  in  this  opinion. 
That  synizesis  is  an  artifact  due  t<»  Faulty  fixation  is  an  interpretation 
which,  though  it  may  be  justified  for  certain  cases  in  which  the  contrac- 
tion may  be  very  slight  or  absent,  is  doI  of  general  application.  Fixa- 
tion often  serves  to  accentuate  the  appearance  of  contraction,  bul  the 
characteristic  synizesis  figure  has  been  observed  widely  enough  in  faith- 
fully preserved,  and  even  in  living  material  to  make  it  evident  that  we 
are  dealing  here  with  a  normal  feature  of  the  heterotypic  prophat  It 
may  not,  however,  be  of  universal  occurrence. 

R.  Hertwig  (1908)  came  to  the  conclusion,  as  a  deduct  ion  from  his 
theory  of  the  nucleoplasmic  relation,  that  the  phenomena  of  the  hetei 
typic  prophase  represent  an  abortive  mitosis:  the  disturbed  nucleoplasmic 
balance  is  restored  to  the  normal  by  a  multiplication  of  chromat  in  without 
an  actual  mitosis,  the  process  taking  the  form  of  the  changes  peculiar  to 
the  heterotypic  prophase.  This  view  of  Hertwig,  which  was  denied  by 
Gregoire  (19096),  is  supported  by  Kingsbury  and  Hirsch  (1912),  who 
state: 

"According  to  this  view,  on  the  one  hand,  synizesis  represents  'an  attempt 
on  the  part  of  the  spermatogonia  to  divide  again — which  fail-:  while  on  the  other 
hand,  the  reputed  conjugation  of  chromosomes  occurring  at  about  this  time  i- 
but  the  imperfect  fission  and  subsequent  fusion  of  daughter  chromosomes  of  Buch 
abortive  division." 

The  above  quoted  authors  regard  synizesis  and  synapsis  as  indicat  i' 
of  the  onset  of  degeneration.  In  this  conclusion  they  are  supported  by 
Kingery  (1917),  who,  in  his  investigation  of  the  white  mouse,  finds 
synizesis  in  the  primitive  germ  cells  which  degenerate,  but  not  in  the 
definitive  germ  cells.  Observations  of  a  similar  nature  were  made  by 
Wodsedalek  (1916)  in  the  mule.  If,  as  Kingery  (1917)  and  Popoff 
(1908)  point  out,  the  "heterotypic'1  changes  are  due  to  degeneration, 
they  should  be  found  in  abnormal  somatic  cells.  Marcus  1907  .  in 
fact,  had  observed  a  contraction  similar  to  that  of  synizesis  preceding 
degeneration  in  the  cells  of  the  thymus  gland.  Nemec  *  r.»o:>i  and  Kemp 
(1910)  also  found  that  in  the  cells  of  roots  treated  with  chloral  hydrate 
the  nuclei  come  to  have  an  abnormally  high  number  of  chromoson 
("syndiploid  nuclei"),  this  number,  according  to  Nemec,  being  gradually 
restored  to  the  normal  during  tin1  subsequent  mite--,  which  -hew 
phenomena  of  a  heterotypic  nature.  Strasburger  1 191 1  I,  while  agreeing 
with  Nemec  that  the  syndiploid  condition  gradually  disappears,  denied 
that  any  truly  heterotypic  phenomena  are  concerned.  The  'hetei 
typic"  changes  observe*  1  by  Xemee  he  held  to  be  only  peculiar  vegetative 
mitoses  with  a  superficial  resemblance  to  genuine  reduction  divisioi 

Nemec's  conclusion  regarding  a  reduction  in  chloralized  vegetative 
cells  is  also  contradicted  by  Sakamura  (1920),  who  has  made  a  particu- 
larly exhaustive   study   of   these    phenomena.     Sakamura    finds    that    a 


260  INTRODUCTION  TO  CYTOLOGY 

variety  of  agencies/including  chloral  hydrate,  benzene  vapor,  ether,  chlo- 
roform, and  the  gall-producing  secretions  of  Heterodera,  may  be  employed 
to  bring  about  aberrations  of  the  mitotic  process.  After  the  chromosomes 
are  divided  and  partially  distributed  they  may  be  reorganized  in  a  single 
:'didiploid':  nucleus.  In  other  cases  the  chromosomes  may  reorganize 
as  two  or  more  nuclei  with  various  chromosome  numbers,  and  these  may 
often  fuse  to  form  "syndiploid"  nuclei.  To  all  the  kinds  of  stimuli  applied 
the  chromosomes  react  by  becoming  shorter  and  thicker,  and  thus  appear 
like  heterotypic  chromosomes.  Furthermore,  latent  or  obscure  constric- 
tions are  rendered  more  conspicuous,  so  that  some  of  the  split  chromo- 
somes appear  like  chromosome  tetrads.  Sakamura  shows  that  these  false 
tetrads  do  not  represent  heterotypic  phenomena  in  any  true  sense:  they  are 
merely  the  result  of  the  response  of  split  and  constricted  chromosomes  to 
the  abnormal  conditions  induced  in  the  cell,  and  have  nothing  to  do  with 
any  reducing  process.  No  such  autoregulative  reduction  occurs  in  these 
didiploid  cells,  their  gradual  decrease  in  relative  number  being  due  to 
their  lowered  capacity  for,  and  rate  of,  division. 

Child  (1915)  emphasizes  the  physiological  significance  of  maturation, 
and  shows  that  the  heterotypic  phenomena  are  associated  with  a  low 
metabolic  rate  in  the  cells,  that  they  may  occur  occasionally  in  other  cells 
having  a  low  rate,  and  that  they  can  be  induced  artificially  with  narcotics 
as  Nemec  stated. 

All  of  these  observations  are  interesting  in  that  they  indicate  the 
nature  of  some  of  the  physiological  changes  occurring  at  the  time  of  matur- 
ation. The  description  of  the  heterotypic  phenomena  upon  which  will 
be  based  our  ultimate  interpretation  of  its  significance,  will  not  be  com- 
plete until  the  physiological  as  well  as  the  morphological  changes  have 
been  exhaustively  examined  and  correlated.  But  because  it  has  been 
found  that  the  onset  of  the  meiotic  process  is  associated  with  a  lowering  of 
the  rate  of  metabolism  which,  if  continued,  may  result  in  degeneration; 
or  because  appearances  similar  to  those  of  the  heterotypic  prophase  may 
occur  in  other  cells  with  disturbed  metabolism;  it  does  not  at  all  follow  that 
the  heterotypic  phenomena  are  at  bottom  phases  of  a  degeneration 
process,  or  that  they  have  no  other  significance  in  the  normal  life  cycle. 
These  phenomena  occur  almost  universally  throughout  the  whole  world 
of  living  organisms  at  a  very  critical  stage  in  the  life  cycle  and  lead  to 
significant  results  with  a  high  degree  of  regularity.  The  lowered  rate  of 
metabolism  accompanying  them  offers  in  the  vast  majority  of  cases  no 
check  to  the  normal  functioning  of  the  products  of  the  maturation  di- 
visions. It  therefore  seems  more  reasonable  to  regard  the  observed 
degeneration  as  a  secondary  effect  that  may  occasionally  set  in  during  the 
normal  heterotypic  prophases  because  the  metabolic  rate  is  already  at  a 
relatively  low  level  at  that  time,  than  to  look  upon  the  heterotypic 
changes  as  a  part  of  a  degeneration  process  which  is  only  exceptionally 


THE  REDUCTION  OF  THE  CHROMOSOMES  261 

completed — unless,  indeed,  all  changes  in  the  organism  which  are  accom- 
panied by  a  fall  in  the  metabolic  rate  be  regarded  as  degenerative  in 

character. 

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1914.     The  behavior  of  the  chromatin  in  the  meiotic  divisions  of  l         Fdba.     [bid. 

28 :  633-642.     pis.  43,  44. 
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264  INTRODUCTION  TO  CYTOLOGY 

Fraser,  H.  C.  I.  and  Brooks,  W.  E.     1909.      Further  studies  in  the  cytology  of  the 

ascus.     Ann.  Bot.  23 :  538-549. 
Fraser,   H.  C.  1.  and  Snell,  J.     1911.     The  vegetative  divisions  in   Vicia  faba. 

Ann.  Bot.  25 :  845-855.     pis.  62,  63. 
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1-34.     pis.  1-3. 

1909.  The  behavior  of  chromosomes  in  (Enothera  lata  X  0.  gigas.    Ibid.  48 .  179-199. 
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1911.  The  mode  of  chromosome  reduction.     Ibid.  51:  321-344. 

Giglio-tos,  E.  e  Granata,  L.     1908.     I  mitocondrii  nelle  cellule  seminali  di  Pam- 

phagus  marmoratus,  Burm.     Biologica  2 :  No.  4. 
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1905.  Les  resultats  acquis  sur  les  cineses  de  maturation  dans  les  deux  regnes. 
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1907.     La  formation  des  gemini  heterotypiques  dans  les  vegetaux.     Ibid.  24:  369- 

420.     pis.  2. 
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245-285.     pis.  2. 
1909b.     Les    phenomenes    de    l'etape    synaptique    represent-ils   une   caryocinese 

avortee?     Ibid.  25 :  87-99. 

1910.  Les  cineses  de  maturation  dans  les  deux  regnes.     L'unite  essentielle  du  pro- 
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1912.  La   verite    du    schema   heterohomeotypique.     Compt.    Rend.    Acad.    Sci. 
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Gregory,  R.  P.     1904.     Spore  formation  in  leptosporangiate  ferns.     Ann.  Bot.  18: 

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Belg.  44:  109-196. 


THE  REDUCTION  OF  THE  CHROMOSOMES  265 

Haecker,  V.     1S90.     Ceber  die  Reifungsvorgange  bei  Cyclops.     Zoo]    \nz  18:551- 

558.     1  fig. 

1892.     Die  Eibildung  bei  Cyclops  und  CarUhocamptus.     Zool.  Jahrb    5-  211-f,l^ 

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14-17. 
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266  INTRODUCTION  TO  CYTOLOGY 

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268  INTRODUCTION  TO  CYTOLOGY 

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crossing  over.     Science  52 :  82-84. 
Nemec,  B.     1903.     Ueber  die  Einwirkung  des  Chloralhydrats  auf  die  Kern-  und 

Zell-teilung.     Jahrb.  Wiss.  Bot.  39:  645-730.     figs.  157. 
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45:31-37.     pi.  5. 
Nothnagel,  M.     1916.     Reduction  divisions  in  the  pollen  mother-cells  of  Allium 

Iricoccum.     Bot.  Gaz.  61:  453-476.     pis.  28-30.     fig.  1. 
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Ber.  Deu.  Bot.  Ges.  34 :  223-227. 
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Equisetum.     Jahrb.  Wiss.  Bot.  30:  159-168.     pis.  1,  2. 
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Dikotylen.     Jahrb.  Wiss.  Bot.  42:  121-153.     pis.  6,  7. 

1906.  The  morphology  of  the  ascocarp  and  spore  formation  in  the  many-spored 
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Bot.  23:  19-61.     pis.  1-3. 


THE  REDUCTION  OF  THE  CHROMOSOMES  269 

Paulmier,  F.  C.     1898.    Chromosome  reduction  in  the  Hemipters.     Anat.  Vnz  14: 
514-520.     figs.  19. 
1899.     The  spermatogenesis  of  Anasa  triads.     Jour.  Morph.  16:  SuppL  223  272. 
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angiosperms.     Bull.  Torr.  Bot.  Club  40:  575-590. 
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Zeitschr.  Wiss.  Zool.  67:  97-185.     pis.  7-9. 
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Rosenberg,  O.     1907.     Zur  Kenntniss  der  praesynaptischen  Entwicklungsphasen 
der  Reduktionsteilung.     Svensk.  Bot.  Tidskr.  1. 
1909a.     Zur    Kenntniss   von    den   Tetradenteilungen  der   Compositen.     Ibid.    3: 

64-77.     pi.  1. 
19096.     Cytologische  und  morphologische  Studien  an  Drosera  longifolia  X  rotundir 

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1918.     Chromosomenzahlen     und     Chromosomendimensionen     in     der     Gattung 
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Anat.  Anz.  7:  107-158.     figs.  6. 

1893.  Die  Chromatinreduktion  bei  der  Reifung  der  Sexualzellen.     Ergeb.  d.  Anat. 
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1894.  Zur  Eireifung  der  Copepoden.     Ibid.,  Anat.  Hefte.  4:  261-352.     pis.  21   25 
Sabaschnikoff,  M.     1897.     Beitrage  zur  Kenntnis  der  Chromatinreduktion  in  der 

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131-147.     pi.  2. 
1915.     Ueber  die  Einschnurung  der  Chromosomen  bei  Vicia  Faba  L.  [bid.  29:  287 

300.     pi.  13.     figs.  12. 
1920.     Experimentelle  Studien  liber  die  Zell-  und   Kernteilung   mil    besonderei 

Riicksicht  auf  Form,  Grosse  und  Zahl  der  Chromosomen.     Jour.  Coll.  Bei.  Imp. 

Univ.     Tokyo  39:  pp.  221.     pis.  7. 
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Schleip,  W.     1906.     Die  Entwicklung  der  Chromosomen  im  Ei  von  Planaria  ij<>no- 

cephata  Dug.     Zool.  Jahrb.  (Anat.  Abt.)  23:  367  380.     pis   23,  24. 
1907.     Die  Samenreifung  bei  Planarien.     Ibid.  21:  129   174.     pis.  11.  IV 
Schellenberg,   A.     1911.     Ovogenese,    Eireifung   und    Befruchtung   von    Fatciola 

hepatica,  L.     Arch.  Zellf.  6:  443-484.     pis.  24  26. 
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28. 
Schreiner,  A.  u.  K.  E.     1904.     Di«'  Eleifungsteilungen  bei  den  Wirbeltieren.     An.it 

Anz.  24:  561-578.     figs.  24. 


270  INTRODUCTION  TO  CYTOLOGY 

1905.  Ueber  die  Entwicklung  der  mannlichen  Geschlechtszellen  von  Myxine  glu- 
tinosa  (L.).     Arch.  d.  Biol.  21:   183-355.     pis.  5-14. 

1906.  Neue  Studien  uber  die  Chromatinreifimg  der  Geschlechtszellen.     Arch.  d. 
Biol.  22:  1-70,  pis.  1-3;  419-492,  pis.  23-26;  Anat.  Anz.  29:  465-479.     figs.  17. 

Sharp,   L.   W.     1913.     Somatic  chromosomes  in    Vicia.     La  Cellule  29:   297-331. 

pis.  2. 
1914.     Maturation  in  Vicia.     (Prelim.  Note).     Bot.  Gaz.  57:  531. 
1920a.     Mitosis  in  Osmunda.     (Review).     Ibid.  69:  88-91. 
19206.     Somatic  chromosomes  in  Tradescaniia.     Am.  Jour.  Bot.  7:  341-354.     pis. 

22,  23. 
Stevens,  N.  M.     1903.     On  the  ovogenesis  and  spermatogenesis  of  Sagitta  bipunctata. 

Zool.  Jahrb.  18:  227-240.     pis.  20,  21. 
1908.     A  study  of  the  germ  cells  of  certain  Diptera,  with  reference  to  the  hetero- 

chromosomes  and  the  phenomena  of  synapsis.     Jour.  Exp.  Zool.  5:  359-374. 

pis.  4. 
1911.     Further  studies  on  the  heterochromosomes  of  the  mosquitoes.     Biol.  Bull. 

20:109-120.     figs.  38. 
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2.     See  Biol.  Centr.  31 :  257-320.     pis.  1-3.     1911. 
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einem  Anhang  uber  Befruchtung.     Hist.  Beitr.  1.     pp.  258.     pis.  3. 
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281-316. 
1897.     Kerntheilung  und  Befruchtung  bei  Fucus.     Jahrb.  Wiss.  Bot.  30:  351-374. 

pis.  27,  28. 
1900.     Ueber  Reduktionsteilung,  Spindelbildung,  Centrosomen  und  Cilienbildner 

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18 :  587-614.     figs.  9. 
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1907.  Ueber  die  Individuality  der  Chromosomen  und  die  Propfhybriden-Frage. 
Ibid.  44 :  472-555.     pis.  5-7. 

1908.  Chromosomenzahlen,  Plasmastrukturen,  Vererbungstrager  und  Reduktions- 
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1910.  Ueber  geschlechtsbestimmende  Ursachen.     Ibid.  47:  427-520.     pis.  9,  10. 

1911.  Kernteilungsbilder  bei  der  Erbse.     Flora  102:  1-23.     pi.  1. 

Sutton,  W.  S.     1902.     On  the  morphology  of  the  chromosome  group  in  Brachystola 

magna.     Biol.  Bull.  4:  24-39.     figs.  11. 
Svedelius,  N.     1914a.     Ueber  die  Tetradenteilung  in  den  vielkernigen  Tetraspor- 

angiumanlagen  bei  Nitophyllum  punctatum.     Ber.   Deu.   Bot.   Ges.  32 :  48-57. 

pi.  1.     1  fig. 
19146.     Ueber  Sporen  an  Geschlechtspflanzen  von  Nitophyllum  punctatum,  usw. 

Ibid.  32:  106-116.     pi.  2.     1  fig. 
Sykes,  M.  G.     1908.     Nuclear  division  in  Funkia.     Arch.  Zellf.  1:  381-398.     pis. 

8,  9.     1  fig. 
Tackholm,    G.     1914.     Zur   Kenntniss   der    Embryosackentwicklung   von    Lopezia 

coronala  Andr.     Svensk.  Bot.  Tidskr.  8. 
Tahara,    M.     1910.     Ueber  die   Kernteilung   bei   Morus.     Bot.    Mag.    Tokyo   24: 

281-289.     pi.  9. 
Taylor,  M.     1914,  1917.     The  chromosome  complex  of  Culex  pipiens.     Quar.  Jour. 

Micr.  Sci.  60:  377-398.     pis.  27,  28;  62:  287-302.     pi.  20. 


THE  REDUCTION  OF  THE  CHROMOSOMES  271 

Tischler,  G.     1906.     Ueber  die  Entwicklung  des  Pollens  and  der  Tapetenzellea  bei 

Ribes-Hyhriden.     Jahrb.  Wiss.  Bot.  42:  545  578.     pi.   15. 
1916.     Chromosomenzahl, -Form  und -Individuality  in  Pflanzenreich.     Prog.  I 

Bot.  5:  164-284.     (Bibliography). 
Tretjakoff,    D.     1904.     Die   Spermatogenese    bei    Ascaria    megalocephala.     Arch. 

Mikr.  Anat.  65:  383-438.     pis.  22-21.      I  fig. 
Trondle,   A.     1911.     Ueber  die  Reduktionsteilung  in  den  Zygotes  von  Spi 

und  liber  die  Bedeutung  der  Synapsis.     Zeitschr.  f.   Hot.  3:  593  619.      1  pi.  20 

figs.    \ 
Tschernoyarow,  M.     1914.     Ueber  die  Chromosomenzahl  und  besonden  beschaf- 

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411-416.     pi.  10. 
Vandendries,  R.     1913.     Le  nombre  des  Chromosomes  dans  la  BpermatogeDJ 

Polytrichum.     La  Cellule  28:  257-261.     figs.  11. 
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P oly trichum- Ar ten.     Rec.  Trav.  Bot.  Xeerl.  4. 
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pi.  5. 
Vejdowsky   F.     1907.     Neue    Untersuchungen    liber    Reifung    und    Befruchtung. 

Kgl.  Bohm.  Ges.  Wiss.     Prag. 
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von  Voss,  H.     1914.     Cytologische  Studien  an  Mesostoma  Ehrenbergi.     Arch.  Zellf. 

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Jour.  Morph.  30:  527-604.     pis.  9.     1  fig. 
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Flora  106:  393-432.     pis.  6,  7.     figs.  10. 
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pis.  40,  41.     1  fig. 


272  INTRODUCTION  TO  CYTOLOGY 

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34:  78-109. 
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1909.  Mitosis  in  Fucus.     Ibid.  47:  173-197.     pis.  8-11. 

1910.  Chromosomes  in  Osmunda.     Ibid.  49:  1-12.     pi.  1. 

1912.     The  life  history  of  Cutleria.     Ibid.  54:  441-502.     pis.  26-35.     figs.  15. 
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143-172.     pis.  11-14. 


CHAPTER    XII 

FERTILIZATION 

We  have  already  pointed  out  that  reduction  and  fertilization  con- 
stitute the  two  principal  cytological  crises  in  the  life  cycles  of  all  organisms 

reproducing  sexually.  Although  the  first  of  these  processes  was  doI  dis- 
covered until  1883,  some  of  the  grosser  features  of  fertilization  had 
been  made  out  many  years  previously  (Chapter  I).  Bu1  the  central 
feature  of  this  process— the  union  of  the  two  parental  nuclei  was  not 
known  until  1875,  when  O.  Hertwig  discovered  it  in  animals,  Strasburger'e 
parallel  discoveries  in  plants  following  in  1877  (Spirogyra)  and  L884 
(angiosperms).  As  the  finer  details  of  fertilization  and  the  significance 
of  its  results  become  better  understood,  the  aptness  of  Huxley's  18^8 
often  quoted  simile,  in  which  he  compares  the  organism  to  "a  web  <»f 
which  the  warp  is  derived  from  the  female  and  the  woof  from  the  mail 
becomes  increasingly  striking. 

We  shall  first  describe  the  morphology  of  the  fertilization  process 
as  it  is  typically  shown  in  many  animals,  after  which  attention  will  be 
given  to  some  of  its  physiological  aspects.  The  second  half  of  t  he  chapter 
will  be  devoted  to  a  review  of  fertilization  in  the  various  groups  of  the 
plant  kingdom. 

FERTILIZATION  IN  ANIMALS1 

The  Gametes. — The  spermatozoa  of  different  animals  exhibit  a 
surprising  variety  of  form  and  structure  (Fig.  103).  What  may  be 
referred  to  as  the  "typical"  spermatozoon  consists  of  three  fairly  distinct 
parts:  head,  middle  piece,  and  tail  or  flagellum  (Fig.  KM  ,  The  /<<</</ 
represents  the  nuclear  portion  of  the  sperm  cell:  it  consists  almost  wholly 
of  an  extremely  compact  mass  of  chromatin.  It  has  an  envelope  of 
cytoplasm  which  in  few  forms  is  very  conspicuous  and  in  many  is 

scarcely  distinguishable.  Anterior  to  the  nucleus  there  may  be  an 
acrosome,  and  the  end  of  the  head  often  has  the  form  of  a  sharp  point, 
the    perferatorium.     Posterior  to  the  head   is  the  middl    \  this  is 

made  up  of  cytoplasm  in  which  are  located  the  centrosomal  structun 
together  with  chondriosomes  and  other  inclusion-.  Buch  :i-  the  Igi 

bodies."     The  flagellum,  or  tail,   consists   of  a   -lender  axial  filament, 

1  In  the  preparation  of  this  portion  of  the  chapter  the  author  lias  drawn  very  fr»  <•!>• 
upon  Professor  F.  R.  Lillie's  admirable  and  concise  presentation  of  tin-  subject, 
Problems  of  Fertilization  (1919). 

18  -'73 


274 


IN TRODVC TION  TO  C Y TOLOG 1 ' 


which  grows  out  from  the  centrosome  in  the  middle  piece  or  in  some 
cases  apparently  from  the  base  of  the  nucleus,  and  a  cytoplasmic  sheath 
which  usually  extends  not  quite  to  its  end.  The  sheath  sometimes  has 
the  form  of  an  undulating  membrane.  The  spermatozoa  of  crustaceans 
and  nematodes  are  non-flagellate,  and  in  other  groups  various  departures 
from  the  " typical"  form  and  structure  are  known.  A  few  of  the  many 
known  types  are  shown  in  Fig.  103. 


G 


Fig.  103. — Various  types  of  spermatozoa. 
A,  Triton  (salamander).  (After  Ballowitz.)  B,  Nereis  (annelid).  (After  Lillie,  1912.) 
C,  guinea  pig.  (After  Meves.)  D,  Phyllopneuste  (bird).  (After  Ballowitz.)  E,  sturgeon 
(After  Ballowitz.)  F,  Vesperugo  (bat).  (After  Ballowitz.)  G,  Castrada  hofmanni  (turbel- 
larian).  (After Luther.)  H,  Pinnotheres  veterum  (crustacean).  (After  Koltzoff.)  I,Homa- 
rus  (lobster).  (After  Herrick).  J,  Ascaris  (nematode);  a,  apical  body;  n,  nucleus;  r,  "re- 
fractive body."      (After  Scheben.) 


The  ovum  undergoes  nearly  or  quite  all  of  its  elaborate  differentiation 
before  the  maturation  divisions  occur.  Certain  cells  in  the  ovary  gradu- 
ally become  greatly  enlarged  (Fig.  105),  and  during  this  "growth  period" 
the  cytoplasm  may  not  only  differentiate  into  visibly  distinct  regions 
but  may  also  become  stored  with  energy-containing  materials  ("food"), 
which  in  the  case  of  some  animals,  such  as  birds,  is  present  in  relatively 
enormous  amounts.  The  "ovarian  egg"  or  primary  oocyte,  as  the  egg 
cell  is  called  before  the  maturation  mitoses  take  place,  may  have  a  definite 
limiting  membrane  at  its  surface,  but  in  many  forms  this  cannot  be 
demonstrated.     The  nucleus  of  the  primary  oocyte  is  known  as  the 


FERTILIZATION 


275 


germinal  vesicle:    it  is  very  large  and  contains  in  addition  to  its  chromatin 

a  considerable  amount  of  material  which  appears  to  take  do  part  in  the 
formation  of  the  chromosomes  when  division  ensui  After  the  cyto- 
plasmic differentiation  is  complete  and  the  oocyte  has  reached  its  full 
size — even  after  the  spermatozoon  has  entered  In  many 
cases — the  oocyte  nucleus  undergoes  two  divisions  in  rapid 
succession  at  the  periphery  of  the  egg,  which  at  this  point 
buds  off  two  small  nucleated  cells,  the  polar  bodies  (Fig.  L06 
The  first  polar  body  may  or  may  not  divide  again.  The 
details  of  chromosome  behavior  in  these  two  mitoses  have  been 
described  in  the  preceding  chapter.  The  reduced  or  haploid 
number  of  chromosomes  left  in  the  egg  organize  tip 
nucleus  ("female  pronucleus"),  rendering  the  egg  ready  for 
the  sexual  fusion. 


Fig.  104.  Fig.    105. 

Fig.   104. — Diagram    of   typical   flagellate   spermafc 

P,  perferatorium;  A,  acrosomc;   A",   nucleus;    M,   middle   piece;  /.    ixial    fUamci 

cytoplasmic  sheath;  E,  end  piece.     (After  Wilson.) 

Fig.  105. — The  differentiation  of  tin-  oocyte  in  Hyd 

A,  very  young  oocyte  lying  betweeri  ectodermal  cells al  right.      B  .\th 

period,  with  yolk  globules,      X  500.     {After  Downing,  1909. 


The  time  relation  of  the  mat  mat  ion  of  i  hi  and  t he  en1  ranee  of  t  he 

spermatozoon  varies  considerably  in  differenl  animals.     In  echinoderms 
and  some  other  forms  maturation  is  complete. 1  before  the  spermato 
penetrates.     In  some  other  animals  it  proceeds  as  far  as  the  metaphase 
of  the  heterotypic  mitosis  (Chcetopterus,  Cerebratulw    of  the  propha 
the  homceotypic  mitosis  (many  vertebrates),   bul    does  do!  go  further 
unless  penetration  occurs.     In  the  marine  annelid.   A     i     .   finally,   tl 


276 


INTRODUCTION  TO  CYTOLOGY 


germinal   vesicle   undergoes   no   change   unless   the    spermatozoon   has 
entered  the  egg. 


Fig.  106. — Maturation '  and  fertilization  in  Ascaris. 
A,  spermatozoon  about  to  enter  egg.  B,  spermatozoon  inside;  first  maturation  mitosis 
in  progress.  C,  first  maturation  mitosis  completed;  first  polar  body  budded  off.  D, 
second  maturation  mitosis,  forming  second  polar  body;  sperm  nucleus  below.  E,  male  and 
female  pronuclei,  each  with  2  chromosomes,  meeting.  F,  first  cleavage  mitosis,  showing  2 
paternal  and  2  maternal  chromosomes.      (After  Hertwig.) 


Fig.  107. — Fertilization  in  Physa  (snail.)  Sperm  head  and  amphiaster  at  right,  with 
long  fiagellum  extending  toward  left.  Second  maturation  mitosis  in  progress.  (After 
Kostanecki  and  Wierzyski,  1896.) 

The  Fusion  of  the  Gametes. — In  most  cases  the  whole  spermatozoon 
enters  the  egg  (Fig.  107).  In  some  sea  urchins  only  the  head  and  middle 
piece  enter,  while  in  Nereis  the  head  alone  passes  in,  the  middle  piece 


FERTILIZATION  277 

and  tail  being  left  on  the  egg  Burface.  The  process  in  \,  ,  as  de- 
scribed by  LiUie  (1912,  1919)  is  as  foUows.     The         ofth         rmha 

tough  vitelline  membrane,  an  alveolar  cortical  layer,  many  yolk  and  oil 
droplets,  and  a  large  central  germinal  vesicle  (nucleus).  If  many 
spermatozoa  are  present  in  the  vicinity  a  large  number  attach  themselves 

to  the  egg,  but  usually  all  but  one  are  carried  away  by  an  outflow  of  jelly 
from  the  alveola*  of  the  cortical  layer.  This  layer  dow  takes  the  form  of 
a  zone  traversed  by  radial  protoplasmic  plates  representing  the  walls  of 
the  alveolae,  A  transparent  "fertilization  cone"  extends  from  the  inner 
part  of  the  egg  across  this  zone  and  touches  the  membrane  a1  the  poinl 
where  the  spermatozoon  is  beginning  to  penetrate.  The  perforatorium 
pierces  the  egg  membrane  and  becomes  attached  to  the  transparent  cone. 
The  latter  is  now  withdrawn,  carrying  the  head  of  the  spermatozoon 
into  the  egg  with  it.  Thus  it  appears  that  the  initiative  for  the  final 
of  penetration  lies  with  the  egg  rather  than  with  the  spermatozo 
Since  only  the  head  enters  the  egg  in  Nereis  it  seems  clear  thai  the  only 
necessary  portion  of  the  spermatozoon  in  the  actual  union  is  the  nucleus: 
the  middle  piece  and  tail  are  accessory  and  function  only  as  locomotor 
organs. 

The  immediate  visible  effects  of  the  entrance  of  the  sperm  are  seen 
chiefly  in  changes  in  the  appearance  of  the  cortical  region  of  the  __  If 
a  vitelline  membrane  is  present,  as  in  vertebrates,  a  "perivitelline 
space"  usually  appears  between  the  membrane  and  the  egg;  and  this 
space  may  in  some  cases  (frog)  be  great  enough  to  permit  the  rotation 
of  the  egg  within  the  membrane.  In  the  sea  urchin  a  /<  rtilization  n 
brane  is  formed  as  the  result  of  fertilization:  it  first  appears  at  the  point 
where  the  spermatozoon  is  attached  and  spreads  over  the  egg  with  great 
rapidity.  It  seems  probable  that  a  delicate  membrane  already  present  is 
raised  and  thus  made  more  conspicuous.  In  Ascaris,  which  is  parasitic 
in  the  intestine  of  the  horse,  this  membrane  becomes  very  thick  and  later 
acts  as  a  protection  against  the  digestive  juices  of  the  host.  Thes 
cortical  changes  do  not  depend  upon  the  actual  entrance  of  the  sperma- 
tozoon into  the  egg:  in  Nereis  they  occur  before  the  slow  penetration 
can  be  completed,  or  even  if  the  spermatozoon  is  shaken  loose  shortly 
after  penetration  has  begun. 

In  describing  the  remarkable  transformation  undergone  f>v  the 
spermatozoon  within  the  egg  the  behavior  of  its  different  organs  will  for 
the  sake  of  clearness  be  considered  separately. 

The  Nucleus. — Immediately  after  gaining  entrance  to  thi 
108)  the  sperm  head  begins  to  enlarge  and  assumes  the  usual  form  and 
structure  of  a  nucleus.  Meanwhile  it  advances  toward  the  egg  nucleus. 
As  Lillie  points  out,  both  male  and  female  pronuclei  pass  toward  a  posi- 
tion of  equilibrium  in  a  cell  preparing  to  divide  and  consequently  meet : 
the  assumption  of  an  attractive  force  between  them  is  unnecessary.     By 


278 


INTRODUCTION  TO  CYTOLOGY 


the  time  they  meet  the  male  pronucleus  has  usually,  but  not  always, 
become  equal  in  size  and  appearance  to  the  female  pronucleus.  The 
union  of  the  two  pronuclei  to  form  a  fusion  nucleus,  or  sijnkaryon,  usually 


Fig.  108. — Diagram  of  fertilization  and  cleavage  in  an  animal.  It  is  assumed  that 
in  this  case  the  egg  has  undergone  maturation  before  the  penetration  of  the 
spermatozoon. 


mmm 


?mm 


C  M^  r^-^tiM^^^S 


Fig.  109. — Independence  of  parental  chromatin  contributions  in  the  cleavage 

of  the  egg  of  Cryptobranchus. 
A    first  cleavage  mitosis.      B,  C,  prophase  and  metaphase  of  fourth  cleavage  mitosis 
(After  Smith,  1919.) 

occurs  at  once  after  they  meet.  In  a  great  many  cases  there  may  be  no 
actual  fusion  of  the  pronuclei  at  all:  as  they  come  close  to  one  another 
each  passes  through  the  prophase  stages  and  gives  rise  independently  to 


FERTILIZATION  279 

its  group  of  chromosomes,  the  two  groups  arranging  themselves  on  a 
common  spindle  which  organize-  when  the  nuclear  membranes  dissoh 
The  first  cleavage  mitosis  (first  embryonal  division    then  ensues,  and 
the  two  daughter  nuclei  receive  longitudinal  halves  of  each  and  every 
chromosome.     Thus  in  the  act  of  ferl  ilizai  ion,  in  bol  h  animals  and  plant 
each  parent  furnishes  the  offspring   with  a  haploid  sit  of  chromosome 
the  two  intermingled  sets  constituting  the  diploid  set  of  th(  m  w  dual. 

Since  every  chromosome  divides  equatio  natty  at  every  subsequent  somatic 
mitosis,  every  cell  of  the  body  receives  half  of  its  chromosome  complement 
from  each  parent.  The  cardinal  importance  of  this  fad  in  connection 
with  current  theories  of  heredity  will  be  apparent  in  subsequent  chapters. 

The  two  groups  of  chromoosmes,  paternal  and  maternal,  can  often  be 
distinguished  not  only  on  the  spindle  of  the  first  cleavage  division,  but 
in  several  divisions  thereafter.  As  examples  may  be  cited  Cyclops 
(Ruckert  1895;  Hsecker  1895),  Crepidula  (Conklin  1901),  and  Crypto- 
branchus  (Smith  1919)  (Fig.  109).  This  phenomenon  is  especially  evident 
in  hybrids  (p.  160).  There  is  much  reason  to  believe  that  the  chro- 
matins of  the  two  parents,  although  intermingled  in  the  nuclei  of  the 
offspring,  never  actually  fuse,  unless  it  is  at  the  time  of  synapsis  in  the 
next  maturation;  and  it  has  already  been  pointed  out  (Chapter  XI  !  thai 
they  may  not  fuse  even  then.  This  fact  also  has  an  important  bearing  on 
the  chromosome  theory  of  heredity. 

The  Centrosome.— (See  Wilson  1900,  pp.  208  fT.)  Shortly  after  the 
entrance  of  the  spermatozoon  into  the  egg  (Figs.  106-108)  an  aster  devel- 
ops at  the  base  of  the  sperm  head,  and  in  the  aster  a  centrosome  appea 
Since  the  centrosome  thus  arises  in  the  position  of  the  middle  piece,  and 
since  the  centrosome  of  the  spermatid  is  included  in  the  middle  piece  dur- 
ing spermatogenesis,  a  widely  accepted  theory  lias  been  that  the  newly 
appearing  centrosome  is  in  reality  that  of  the  spermatid.  \\ 'hat ever  its 
origin,  it  soon  divides  to  form  the  two  which  fund  ion  in  I  he  firsi  cleavi 
mitosis.  These  facts  had  much  to  do  with  the  formulation  of  a  theory  of 
fertilization  set  forth  by  Boveri  (1887,  1891),  who  was  much  impressed  by 
the  conspicuous  part  played  by  the  centrosomes  in  cell-division.  Accord- 
ing to  Boveri's  theory  the  egg  is  not  able  to  undergo  division  because  of 
the  lack  of  any  centrosome  to  initiate  the  process,  while  the  Bpermatoaoon 
has  a  centrosome  but  not  sufficient  cytoplasm  in  which  toact.  Through 
the  union  of  the  gametes  all  the  organs  necessary  for  division  are  brought 
together  and  cleavage  proceeds.  This  theory  has  recently  been  recalled 
by  Walton  (1918)  in  his  work  on  Ascaris  nun's. 

Another  early  view  of  the  origin  of  the  cleavage  centrosomes  was  that 
of  van  Beneden  (1887)  and  Wheeler  (1895,  L897  ,  who  believed  them  to  be 

the  centrosomes  of  the  egg  cell. 

The  theory  that  the  cleavage  centrosomes  arise  from  both  egg  and 
spermatozoon  is  of  some  historic  interest.     It   was  sug  d  by  Rabl 


280  INTRODUCTION  TO  CYTOLOGY 

(1889)  that  if  the  centrosome  is  a  permanent  cell  organ  the  conjugation  of 
the  gametes  must  involve  not  only  a  union  of  nuclei  but  also  a  union  of 
centrosomes  (Wilson,  p.  210).  Fol  (1891),  in  his  work  on  echinoderm 
eggs,  thought  that  he  observed  just  such  a  process,  which  he  termed  "The 
Quadrille  of  the  Centers."  The  egg  centrosome  and  the  centrosome 
brought  in  by  the  spermatozoon  were  both  supposed  to  divide,  the  prod- 
ucts then  fusing  in  pairs  to  form  the  two  cleavage  centrosomes.  A  simi- 
lar thing  was  reported  by  certain  other  investigators,  but  none  of  the  cases 
stood  the  test  of  later  work.  Another  theory  now  abandoned  was  that 
advanced  by  Carnoy  and  Lebrun  (1897),  who  also  attempted  to  derive  one 
centrosome  from  each  gamete.  The  cleavage  centrosomes  were  thought 
to  arise  de  novo  and  separately,  one  inside  each  pronucleus,  to  migrate 
thence  into  the  cytoplasm. 

Much  less  confidence  is  now  placed  in  the  persistence  of  the  spermatid 
or  egg  centrosomes  through  the  fertilization  stages.  Since  the  middle 
piece,  which  is  thought  to  contain  the  centrosome,  does  not  enter  the  egg 
at  all  in  Nereis,  it  seems  probable  that  the  male  nucleus  in  some  way 
induces  the  formation  of  asters  and  centrosomes  by  the  egg  cytoplasm. 
Lillie  found  that  even  a  portion  of  the  sperm  head  will  bring  about  this 
effect.  In  Unio  (Lillie  1897,  1898)  and  Crepidula  (Conklin  1897)  it  seems 
not  unlikely  that  each  pronucleus  causes  the  formation  of  one  cleavage 
centrosome.  In  the  sea  urchin  Wilson  (1901)  concluded  that  the  cleav- 
age centrosomes  in  all  probability  arise  by  the  division  of  one  which  orig- 
inates de  novo  at  the  nuclear  membrane.  In  almost  every  case  there  are 
gaps  in  the  known  history  of  the  centrosome  in  fertilization,  and  it  seems 
very  doubtful  whether  the  cleavage  centrosomes  are  continuous  with 
those  of  either  gamete.  This  conclusion  is  supported  by  the  fact  that  the 
formation  of  asters  with  centrosomes  in  the  egg  cytoplasm  can  be  arti- 
ficially induced  by  treating  the  eggs  with  certain  chemicals,  such  as  weak 
MgCl2.  It  is  possible  that  the  spermatozoon  carries  a  substance  which 
brings  about  centrosome  formation  in  a  similar  way.  However  this  may 
be,  the  importance  of  the  centrosome  undoubtedly  lies  in  its  relation  to 
cleavage  rather  than  to  fertilization. 

Cytoplasm  and  Chondriosomes. — In  some  cases  (Nereis)  no  cytoplasm 
can  be  shown  to  enter  the  egg  with  the  spermatozoon,  whereas  in  others 
(Ascaris)  a  relatively  large  amount  is  brought  in.  Its  great  indefiniteness 
in  behavior  makes  it  seem  probable  that  it  has  no  special  significance  in 
the  fertilization  process. 

The  importance  of  the  chondriosomes  in  fertilization  has  been  empha- 
sized by  Meves  (1911,  1915),  who  finds  that  many  of  these  bodies  are 
present  in  the  large  cytoplasmic  mass  accompanying  the  sperm  nucleus 
in  Ascaris,  and  that  they  mingle  with  the  chondriosomes  of  the  egg. 
Meves  (1908,  1915,  1918),  together  with  other  writers,  accordingly  thinks 
that  they  are  concerned  in  the  transmission  of  certain  hereditary  char- 


FERTILIZATinx 


281 


acters.  Several  observations,  however,  fail  <<>  harmonize  with  this  view. 
In  some  echinoderms  the  middle  piece,  which  contains  the  chondriosomal 
material,  is  not  distributed  to  the  daughter  cells  during  the  cleavaj 

divisions,  but  remains  in  one  of  them.  It  is  unlikely  thai  hereditary 
material  would  behave  in  this  manner.  Furthermore,  in  the  worm. 
Nereis,  the  middle  piece  does  not  even  enter  the  egg.  In  Peripatus 
(Montgomery  1912)  the  chondriosomal  mass  is  thrown  out  of  the 
spermatid.  Wildman  (1913)  points  out  that  in  Ascaris  also  the  chon- 
driosomes  may  be  largely  lost  during  spermatogenesis.  Although  their 
almost  constant  presence  in  the  spermatozoon  indicates  that  they  have 
a  special  physiological  role  comparable  to  that  in  other  cells,  there  is  a& 
yet  no  adequate  reason  to  regard  them  as  important  in  the  transmission 
of  the  factors  of  inheritance. 


Fig.  110. — Ohromidiogamy  in  Arcella.     (From   Minchin,  after  Swarasewaky.) 

Somewhat  modified. 

Fertilization  in  Protozoa. — Among  the  protozoa  there  arc  ton  ml 
several  modes  of  sexual  union  very  different  from  that  described  above  for 
the  higher  animals.  Four  illustrative  cases  may  be  cited  Bee  Minchin 
1912). 

In  Arcella,  a  rhizopod  with  a  hemispherical  shell,  the  protoplast  1 
two  "primary  nuclei"  and  many  scattered  chromidia.     Two  individuals 
come  together  with  their  shell  openings  opposite  one  another  I  Fig.  1 1' 
and  the  protoplasm  of  one  flows  almost  entirely  into  the  other  shell. 
where  it  mingles  with  that  of  the  other  individual.     The  primary  nuclei 
degenerate,  while  the  chromidia  become  divided  up  into  fine  granul< 
The  protoplasm  with  the  fine  chromidia  now  becomes  evenly  distributed 
in  the  two  shells,  after  which  the  latter  separate.     In  each  individual  the 
chromidia  now  form  "secondary  nuclei;"  around  these  are  organised 
uninucleate  amcebulse  which  escape  from  the  shell  and  give  rise  to  Dew 
Arcella  individuals.     This  process,  in  which   the  chromatin  chiefly  con- 


282 


INTRODUCTION  TO  CYTOLOGY 


cerned  is  that  of  the  chromidia  and  not  that  of  the  larger  nuclei,  is  quite 
rare  and  is  known  as  chromidiogamy .  Arcella  also  has  the  other  form  of 
sexual  union,  karyogamy. 


sh:C\ 


Fig.   111. — Copulation  in   Actinophrys  sol.      X  850.      (From   Minchin,   after 

Schaudinn.) 

In  Actinophrys  sol  (Schaudinn  1896)  (Fig.  Ill)  two  individuals,  each 
with  a  single  nucleus,  approach  each  other  and  become  enclosed  in  a 
common  cyst.  In  each  of  them  the  nucleus  now  undergoes  two  pre- 
liminary mitotic  divisions,  at  each  of  which  a  small  "reduction  nucleus" 
is  expelled  from  the  body  in  a  manner  very  similar  to  the  expulsion  of 
chromatin  into  the  polar  bodies  of  higher  animals.     The  two  individuals, 


Fig.  112. — Autogamy  in  Amoeba  albida.     (From  Minchin,  after  Nagler.) 

or  gametes,  as  they  may  now  be  called,  fuse  completely,  their  nuclei 
uniting  to  form  a  synkaryon.  Soon  the  synkaryon  divides  mitotically, 
this  being  followed  by  the  division  of  the  cell  to  form  two  individuals 
which  escape  from  the  cyst  and  resume  the  vegetative  state. 

In  Amoeba  albida  (Nagler  1909)  (Fig.  112)  a  peculiar  process  known  as 
autogamy  occurs  while  the  organism  is  in  the- encysted  state.     The  single 


FERTIUZATIOX 


283 


nucleus  divides,  forming  a  large  vegetative  nucleus  and  a  smaller  gener- 
ative nucleus.  The  former  moves  to  the  surface  of  the  cell  and  degener- 
ates, while  the  latter  gives  rise  to  the  gamete  nuclei  in  the  following 
manner.  After  becoming  elongated  and  incompletely  divided  it  buds 
off  four  "reduction  nuclei" — two  from  one  end  and  then  two  from  the 


Fig.   113.     Conjugation   in  Paramcecium.     (After   Mvrgan,    19] 

other.  It  then  completes  its  division  to  form  the  two  gamete  nuclei, 
or  pronuclei,  while  the  four  reduction  nuclei  degenerate.  Ifter  lying 
apart  for  some  time  the  two  pronuclei  approach  each  other  and  fuse; 
sexual  union  thus  takes  place  between  Bister  nuclei. 

The  complicated  nuclear  behavior  occurring  at  t  he  t  ime  of  conjugal  ion 
m.  Paramcecium  caudatum  (Fig.  113)  may  best  be  described  in  the  words 


284  INTRODUCTION  TO  CYTOLOGY 

of  Wilson  (1900,  pp.  224-226)0  In  "Paramecium  caudatum,  which 
possesses  a  single  macronucleus  and  micronucleus,  .  .  .  conjugation  is 
temporary  and  fertilization  mutual.  The  two  animals  become  united 
by  their  ventral  sides  and  the  macronucleus  of  each  begins  to  degenerate, 
while  the  micronucleus  divides  twice  to  form  four  spindle-shaped  bodies. 
Three  of  these  degenerate,  forming  the  ' corpuscles  de  rebut,'  which 
play  no  further  part.  The  fourth  divides  into  two,  one  of  which,  the 
'female  pronucleus,'  remains  in  the  body,  while  the  other,  or  'male 
pronucleus,'  passes  into  the  other  animal  and  fuses  with  the  female 
pronucleus.  Each  animal  now  contains  a  cleavage-nucleus  equally 
derived  from  both  the  conjugating  animals,  and  the  latter  soon  separate. 
The  cleavage-nucleus  in  each  divides  three  times  successively,  and  of 
the  eight  resulting  bodies  four  become  macronuclei  and  four  micronuclei. 
By  two  succeeding  fissions  the  four  macronuclei  are  then  distributed, 
one  to  each  of  the  four  resulting  individuals.  In  some  other  species 
the  micronuclei  are  equally  distributed  in  like  manner,  but  in  P.  caudatum 
the  process  is  more  complicated,  since  three  of  them  degenerate,  and 
the  fourth  divides  twice  to  produce  four  new  micronuclei.  In  either 
case  at  the  close  of  the  process  each  of  the  conjugating  individuals 
has  given  rise  to  four  descendants,  each  containing  a  macronucleus  and  a 
micronucleus  derived  from  the  cleavage-nucleus.  From  this  time  forward 
fission  follows  fission  in  the  usual  manner,  both  nuclei  dividing  at  each 
fission,  until,  after  many  generations,  conjugation  recurs." 

The  Physiology  of  Fertilization. — The  principal  results  of  fertiliza- 
tion are  two:  the  activation  of  the  egg,  and,  in  dioecious  organisms, 
biparental  heredity.  Both  of  these  have  their  physiological  as  well  as 
their  morphological  aspects,  and  in  the  present  section  the  first  of  them 
will  be  considered  with  special  reference  to  its  physiology.  What  is  the 
nature  of  the  physiological  reactions  through  which  the  development  of 
the  egg  is  initiated?  In  the  terms  used  by  Child  (1915),  how  does 
fertilization  bring  about  the  rejuvenation  of  the  egg,  which  is  a  physio- 
logically old  cell?  The  attack  upon  this  problem  has  been  carried  out 
along  two  main  lines:  by  a  study  of  artificial  parthenogenesis  and  by  a 
direct  analysis  of  the  chemical  constitution  of  the  gametes  at  these  stages. 

Artificial  Parthenogenesis. — This  line  of  attack  has  been  followed 
particularly  by  Loeb,  who  has  found  a  number  of  methods  by  which  the 
parthenogenetic  development  of  unfertilized  eggs  may  be  artificially 
induced.1  As  stated  in  the  foregoing  pages,  the  first  externally  visible 
effect  of  fertilization  is  in  many  animals  the  formation  of  a  fertilization 
membrane.  The  formation  of  this  membrane,  which  seems  to  be  a 
condition  necessary  to  the  future  development  of  the  egg,  Loeb  was  able 
to  induce  in  the  California  sea  urchin  by  placing  the  eggs  for  2  minutes 
in  a  solution  made  up  of  50  c.c.  of  sea  water  and  3  c.c.  of  a  tenth-normal 
1  For  a  convenient  summary  of  such  methods  see  Harvey  (1910). 


FERTILIZATION  285 

fatty  acid  (butyric,  propionic,  or  valerianic),  and  then  back  into  pure 
sea  water:  the  membrane  then  forms  by  a  cytolysis  of  the  cortical  laj 
of  the  egg.     Although  in  some  forms  (starfish)  this  one  treatmenl    is 
sufficient  to  bring  about  successful  development,   in   mosl  sea 

urchin)  the  eggs  become  sickly  and  die.  Loeb  found  thai  this  sickli- 
ness may  be  prevented,  allowing  normal  development,  by  either  of  two 
second  treatments.  If,  after  membrane  formation,  the  eggs  are  placed 
for  20  minutes  in  hypertonic  sea  water  or  other  solution  with  an  osmotic 
pressure  50  per  cent  above  that  of  ordinary  sea  water,  they  will  develop 
normally  when  returned  to  pure  sea  water.  The  same  effect  may  be 
brought  about,  though  not  always  so  successfully,  by  placing  th< 
for  3  hours  in  sea  water  free  from  oxygen,  or  into  sea  water  with  a  trace 
of  KCN.  It  is  therefore  concluded  by  Loeb  that  the  stimulus  to  Buch 
parthenogenetic  development  has  two  phases:  the  inducement  <>!  mem- 
brane formation  by  cytolysis,  and  the  subsequent  effect  of  the  hyper- 
tonic solution.  In  rare  cases  the  first  treatment  alone  is  sufficient  for 
normal  development,  but  in  all  cases  it  at  least  starts  the  egg  into  activity. 
As  a  result  of  these  experiments  Loeb  has  interpreted  the  action  of  the 
spermatozoon  in  normal  fertilization  on  the  assumption  thai  it  carries 
two  substances:  first,  a  lysin  which  brings  about  membrane  format  ion 
by  cytolysing  the  cortical  layer  of  the  egg,  and  which  can  act  even  if 
the  spermatozoon  does  not  enter  the  egg;  and  second,  a  substance  which 
produces  an  effect  similar  to  that  of  the  hypertonic  sea  water  employed 
in  the  experiments.  The  quite  different  explanation  offered  by  Lillie 
will  be  mentioned  further  on. 

How  it  is  that  cytolysis  of  the  cortical  layer  of  the  eg,L>-  brings  about 
activation  Loeb  attempts  to  explain  in  the  following  manner.  A  calcium 
lipoid  compound  forms  a  continuous  layer  just  beneath  the  surface  of  the 
egg,  and  the  solution  of  this  layer  wTould  probably  result  in  the  destruc- 
tion of  the  cortical  emulsion.  It  is  assumed  that  in  this  cortical  region 
there  is  a  catalytic  agent  which  increases  the  metabolism  irate  of  oxida- 
tion, etc.)  of  the  egg.  Following  Warburg  (1914)  Loeb  suggests  thai  the 
cytolysis  releases  the  catalyzer  by  breaking  down  the  cortical  emulsion; 
this  results  in  an  increase  in  the  rate  of  oxidation  and  other  react  ions,  and 
development  proceeds. 

That  the  process  of  activation  is  bound  up  primarily  with  reactions 
occurring  in  the  cortical  region  of  the  egg  is  shown  further  by  the  experi- 
ments of  Guyer  (1907),  Herlant  (1913,  L917),  McClendoD  I  1912),  Loeb 
and  Bancroft  (1913),  and  particularly  Bataillon  (  L910),  who  have  shown 
that  the  egg  of  the  frog  may  be  made  to  develop  by  pricking  it  with  a 
needle,  especially  if  some  blood  enters  the  egg  with  it ;  and  also  by  the 
researches  of  R.  S.  Lillie  (1908,  1915),  who  finds  that  Btarfish  eggs  may 
be  made  to  develop  parthenogenetically  by  exposing  them  to  high 
temperatures  for  definite  periods.     (See  V.  R.  Lillie.  I'M!*.  I  Ihapter  VII.) 


286  INTRODUCTION  TO  CYTOLOGY 

Heilbrunn  (1920)  shows  that  the  egg  of  Cumingia  can  be  induced  to 
undergo  maturation  by  agencies  which  release  the  fluid  cytoplasm  from 
the  restraint  of  the  tough  vitelline  membrane.  If  the  membrane  be 
swollen,  elevated  above  the  egg  surface,  ruptured,  or  removed  the 
maturation  changes  begin  at  once. 

The  sickliness  and  death  of  those  eggs  given  only  the  first  treatment 
Loeb  thought  to  be  due  to  the  continued  action  of  the  cytolytic  agent. 
Against  this  conception  it  is  urged  by  F.  R.  Lillie  that  since  any  activated 
egg  not  developing  normally  cytolyzes  sooner  or  later  from  internal  causes, 
it  is  more  probable  that  the  sickliness  and  death  are  due  to  some  internal 
cause  resulting  from  activation,  and  points  out  that  such  a  conclusion  is 
supported  by  the  cytological  phenomena  in  eggs  activated  by  Loeb's 
method.     To  these  phenomena  we  may  turn  for  a  moment. 

Eggs  which  have  been  given  the  first  treatment  alone  do  not  begin  to 
disorganize  for  many  (12  to  24)  hours.  During  this  period  Herlant  (1917) 
has  observed  the  following  events.  After  the  formation  of  the  mem- 
brane and  a  hyaline  zone,  alterations  cease,  and  the  nucleus  becomes  the 
seat  of  a  series  of  conspicuous  changes.  The  nuclear  membrane  dissolves, 
and  around  the  chromosomes  there  is  formed  a  monaster  (one-poled 
group  of  achromatic  fibers),  but  no  amphiaster  develops.  The  chromo- 
somes divide  but  do  not  separate,  and  although  the  cytoplasm  becomes 
active  no  cytokinesis  ensues.  The  chromosomes  then  return  to  the 
resting  condition.  This  process  is  repeated  several  times,  the  nucleus 
increasing  in  bulk  each  time,  but  it  soon  becomes  very  irregular  and  the 
egg  ultimately  breaks  down  by  general  cytolysis.  The  second  treatment 
(Loeb's  method)  in  some  way  gives  the  egg  the  capacity  to  divide  regu- 
larly. Morgan  (1899)  and  Wilson  had  long  before  shown  that  such 
treatment  with  hypertonic  sea  water  causes  aster  formation  in  the 
unfertilized  sea  urchin  egg.  Herlant  shows  that  one  of  these  asters  and  a 
second  aster  formed  in  connection  with  the  egg  nucleus  together  form  an 
amphiaster,  normal  division  then  ensuing. 

In  the  light  of  these  facts  it  seems  evident  that  the  death  of  the  egg 
after  the  first  treatment  alone  is  not  due  to  the  continued  action  of  the 
cytolytic  agent  employed,  but  rather  to  irregularities  in  the  activation 
processes  aroused  by  the  cortical  changes  in  the  absence  of  a  proper 
coordination  of  nuclear  and  cell  division.  The  second  treatment  pro- 
duces a  regulatory  effect,  partly  through  aster  formation,  resulting  in 
normal  development.  This  recalls  Boveri's  morphological  theory  of 
normal  fertilization. 

Direct  Analysis  of  the  Fertilization  Process. — In  contrast  to  the  theory 
that  the  spermatozoon  contributes  organs  (Boveri)  or  substances  (Loeb) 
necessary  for  the  activation,  Lillie  (1919,  Chapter  VII)  regards  the  egg 
itself  as  an  " independently  activable  system."  "The  egg  possesses  all 
substances  needed  for  activation;  the  spermatozoon  is  an  inciting  cause 


FERTILIZA  TION  287 

of  those  reactions  within  the  egg  system  upon  which  development 
depends."  As  a  result  of  his  direct  analysis  of  the  gametes  during  the 
fertilization  period  Lillie  has  identified  a  substance  in  the  egg  which  he 
calls  fertilizin.     This  substance  is  present   in  the  egg  for  a  short  time 

only;  its  formation  usually  begins  at  about  the  time  the  germinal  vesicle 
begins  to  break  down,  and  immediately  after  fertilization  its  production 
ceases,  possibly  through  the  neutralizing  action  of  a  second  substance, 
called  "anti-fertilizinr."  As  a  rule  it  is  only  during  the  period  ;it  which 
fertilizin  is  present  that  spermatozoa  will  enter  the  egg;  the  egg  re- 
mains fertilizable  for  but  a  short  time.  Hence  it  seems  clear  that  it  is 
not  the  fertilization  membrane  that  prevents  the  entrance  of  other 
spermatozoa,  as  Fol  thought,  but  rather  the  physiological  state  of  1  he  egg. 
That  the  protection  is  thus  a  physiological  rather  than  a  mechanical 
one  is  indicated  by  the  fact  that  membraneless  egg  fragment-  without 
fertilizin  are  not  entered  by  spermatozoa. 

Fertilizin  has  two  effects:  it  first  acts  by  causing  an  agglutination 
of  the  spermatozoa  at  the  surface  of  the  egg,  and  later  causes  the  activa- 
tion of  the  egg.  It  may  thus  be  said  to  stand  between  the  spermatozoon 
and  the  activation  reactions  in  the  egg.  Being  present  in  the  egg  secre- 
tion at  a  certain  period  it  binds  the  spermatozoon  to  the  surface  of  t  he  egg 
and  the  spermatozoon,  without  necessarily  penetrating  the  egg  at  all. 
by  means  of  a  substance  which  it  bears  releases  the  activity  of  the 
fertilizin  within  the  egg,  which  results  in  development.  In  brief,  the 
activating  substance  is  already  present  in  the  egg  and  is  not  brought 
to  it  by  the  spermatozoon.  It  may  be  incited  to  activity  by  the  sperm- 
atozoon, but  by  other  agencies  as  well. 

In  concluding  this  sketch  of  the  physiological  features  of  fertiliza- 
tion we  may  state  briefly  the  immediate  physiological  consequences  of  the 
process  as  summarized  by  Lillie  (1919,  Chapter  V).  The  rate  of  oxida- 
tion increases  in  most  cases  in  which  it  has  been  investigated.  In  the  sea 
urchin  egg  (Warburg  1908-1914)  this  rate  increases  as  much  as  six-  or 
seven-fold;  in  Strongylocentrotus,  four-  or  five-fold  (Loch  and  Wasteneys, 
1912,  1913);  in  the  starfish,  apparently  not  at  all.  The  egg  membrane 
becomes  more  permeable  to  oxygen,  CO2,  pigment,  water,  alkalis,  intra- 
vitam  stains,  and  a  number  of  other  substance-.  The  protoplasm  be- 
comes less  fluid  after  fertilization  (Heilbrunn  1915  This  gelation  effect 
Chambers  (1917)  believes  to  center  upon  the  sperm  aster.  The  volume 
of  the  egg  decreases  and  its  electrical  conductivity  rises.  The  most 
conspicuous  chemical  change  is  seen  in  the  loss  of  the  fertilizin.  and  with 
it  the  loss  of  capacity  for  further  fertilization  reaction. 

FERTILIZATION  IN  PLANTS 

Although  the  central  act  of  the  process  of  fertilization  is  regularly 

the    union    of    two    sexually    differentiated    nuclei,    the    morphological 


288 


INTRODUCTION  TO  CYTOLOGY 


features  associated  with  this  fusion  are  more  varied  in  plants  than  in 
animals.     This  is  especially  true  of  the  algae  and  fungi. 

Algae. — In  Ulothrix  fertilization  consists  in  the  complete  union  of 
two  morphologically  similar,  motile  biciliate  gametes  (Fig.  114,  A). 
In  Fucus  the  two  gametes  are  very  dissimilar :  the  male  (spermatozoid)  is 
small,  laterally  biciliate,  and  actively  motile  (Fig.  114,  B),  while  the 
female  (egg),  though  discharged  from  the  oogonium,  is  large  and  passive, 
as  in  all  higher  plants  and  animals.     In  (Edogonium  (Fig.  114,  D,  E)  the 


Fig.  114. — Spermatozoids  of  plants. 

A,  Ulothrix:  1,  gamete;  2,  gametes  fusing  (isogamy) ;  3,  zygospore.  B,  Fucus.  {After 
Guignard.)  C,  Zamia.  (After  Webber.)  D,  bit  of  filament  of  (Edogonium;  spermatozoids 
escaping  from  antheridial  cells  below;  spermatozoid  about  to  enter  egg  above.  {After 
Coulter.)  E,  spermatozoid  of  (Edogonium.  F,  Chara.  (After  Belajeff.)  G,  Onoclea. 
(After  Steil.)  For  figures  of  spermatozoids  of  Blasia,  Polytrichum,  Equisetum,  and  Marsilia, 
see  Figs.  28,  29,  30,  and  32. 


egg  is  not  shed  from  the  cell  which  produces  it,  but  is  fertilized  in  situ, 
a  condition  which  is  retained  in  all  the  higher  plant  groups.  The  sperm- 
atozoid in  this  genus  has  a  crown-like  ring  of  cilia.  In  Spirogyra  (and 
other  Conjugatae)  certain  vegetative  cells,  without  further  morphological 
differentiation,  function  as  gametes.  The  entire  contents  of  such  a  cell 
pass  through  a  conjugation  tube  to  a  similar  cell  in  an  adjacent  filament, 
where  the  two  unite  to  form  the  zygospore.  The  two  nuclei  fuse,  but  the 
chloroplasts  furnished  by  the  contributing '(" male")  gamete  may  event- 
ually degenerate  (Zygnema).     In  Polysiphoiiia  a  non-motile  male  gamete 


FERTILIZAWtX 


289 


(spermatium)  comes  in  contact  with  a  prolongation  (trichogyne)  of  the 
female  sex  organ  (carpogonium).  Solution  of  the  intervening  walla  allows 
the  nucleus  of  the  spermatium  to  pass  into  the  1  richogyne  and  down  to  I  lie 
female  nucleus  in  the  base  of  the  carpogonium.  In  Polysiphonia  we 
have  one  of  the  few  cases  among  lower  plants  in  which  the  fusion  of  the 
sexual  nuclei  has  been  minutely  described.  According  to  Yamanouchi 
(1906)  the  male  nucleus,  by  the  time  it  has  reached  the  female  nucleus, 
has  resolved  itself  into  a  group  of  20  chromosomes  (Fig.  115,  A).     In  i  his 


: 

.'■<■ 

-    • 
..     . 


■ 
i    :■■'".•'■■    "-'■'    *. 

V 


Fig.  115. 

A,  fertilization  in  Polysiphonia.  Group  of  male  chromosomes  about  to  enter  female 
nucleus.  (After  Yamanouchi,  1906.)  B,  fertilization  in  Albugo  Candida.  Female  nucleus 
lying  in  center  of  ooplasm  near  the  "  ccenocentrum  "  (larger  dark  body.  I  A  ntheridial  tube 
about  to  discharge  a  male  nucleus;  another  male  nucleus  in  neck  of  tube.  Additional  nuclei 
in  periplasm  surrounding  the  ooplasm.      (After  Davis,  1900.) 


condition  it  enters  the  female  nucleus  while  the  latter  is  yet  in  the  reticu- 
late state.  Soon  the  female  reticulum  becomes  transformed  into  20 
chromosomes,  which  arrange  themselves  with  the  20  paternal  chromo- 
somes upon  the  spindle  as  the  fusion  nucleus  divides. 

Fungi. — In  the  Phycomycetes  sexual  reproduction  occurs  in  I  wo  princi- 
pal forms,  which  serve  to  divide  the  group  into  two  main  divisions: 
Oomycetes  and  Zygomycetes. 

In  the  Oomycetes  the  cytologica!  phenomens  are  best  known  in  the 
Peronosporales  and  Saprolegniales.  In  the  former  there  is  differentiated 
in  the  oogonium  a  single  large  egg  Into  which  the  contents  of  an  antheri- 
dium  are  discharged  through  a  penetrating  tube.  In  Albugo  Writ  and 
A.  portulaccce  (Stevens  1899,  1901)  the  egg  has  a  large  number  of  nuclei, 

19 


290  INTRODUCTION  TO  CYTOLOGY 

and  after  the  entrance  of  the  antheridial  nuclei  about  100  sexual  fusions 
occur.  In  Albugo  Candida  {Cystopus  candidus)  (Wager  1896;  Davis 
1900),  Peronospora  parasitica  (Wager  1900),  Albugo  tragopogonis  and 
A.  ipomceae  (Stevens  1901)  the  mature  egg  has  but  one  nucleus,  which 
fuses  with  a  single  male  nucleus  discharged  into  the  egg  by  an  antheridium 
(Fig.  115,  B).     In  all  cases  an  oospore  results. 

In  the  Saprolegniales,  as  shown  by  the  researches  of  Davis  (1903, 
1905),  Miyake  (1901),  Trow  (1895-1905),  and  Claussen  (1908),  there  are 
two  general  conditions.  In  Saprolegnia  (Trow,  Davis,  Claussen)  from 
10  to  15  uninucleate  eggs  are  formed  within  an  oogonium.  One  or  more 
antheridia  send  in  conjugating  tubes  and  deliver  a  male  nucleus  to  each 
egg,  in  which  a  single  sexual  fusion  then  occurs.  In  Pythium  (Trow  1901 ; 
Miyake  1901)  a  single  uninucleate  egg  is  produced,  the  fertilization 
process  closely  resembling  that  in  Albugo  Candida. 

In  the  Zygomycetes,  represented  chiefly  bytheMucoracese,  the  sexual 
process  consists  in  the  union  of  the  contents  of  two  similar  (except  oc 
casionally  in  size)  multinucleate  gametangia,  the  result  of  the  fusion 
being  a  zygospore.  As  shown  by  Blakeslee  (1904)  these  two  gametangia 
are  borne  on  the  same  mycelium  in  some  species  ("homothallic"  species), 
whereas  in  other  species  ("heterothallic"  species)  they  are  regularly 
borne  on  different  mycelia,  no  zygospores  being  formed  in  the  latter  spe- 
cies on  a  mycelium  arising  from  a  single  spore.  Owing  to  the  extremely 
minute  size  of  the  nuclei  their  behavior  at  these  stages  is  not  well  known. 
By  some  investigators  (Macormick  on  Rhizopus  nigricans,  1912)  it  is 
held  that  only  one  fusion  occurs,  the  remaining  nuclei  degenerating. 
Others  (Keene  on  Sporodinia  grandis,  1914)  think  it  probable  that  al- 
though some  degeneration  occurs,  the  nuclei  nevertheless  fuse. in  pairs 
in  considerable  numbers.  Until  further  researches  have  been  carried  out 
very  little  of  a  definite  nature  can  be  said  concerning  the  nuclear  history 
of  the  Zygomycetes. 

In  the  Ascomycetes  (see  Atkinson  1915)  the  fusion  of  two  nuclei 
in  the  ascus  was  first  described  for  several  species  by  Dangeard  (1894) 
(Fig.  116,  A),  who  regarded  it  as  a  sexual  fusion  and  the  ascus  as  an  oogo- 
nium. The  matter  soon  became  complicated  when  a  number  of  cytolo- 
gists,  beginning  with  Harper  (1895  etc.),  found  what  they  believed  to  be 
a  nuclear  fusion  at  an  earlier  stage  in  the  life  cycle.  This  fusion  was 
described  as  occurring  (a)  in  the  archicarp  when  fertilized  by  the  contents 
of  an  antheridium  (Harper  on  Sphcerotheca  castagnei,  1895,  1896,  Erisiphe 
1896,  Pyronema  confluens  1900,  and  Phyllactinia  1905;  Blackman  and  Fra- 
ser  on  Sphcerotheca  1905;  Claussen  on  Boudiera  1905) ;  (6)  in  the  archicarp 
when  the  antheridium  is  functionless  or  absent  (Blackman  and  Fraser 
on  Humaria  granulata  1906;  Fraser  on  Lachnea  stercorea  1907;  Welsford 
on  Ascobolus  furfuraceus  1907;  Dale  on  Aspergillus  repens  1909);  or  (c) 
in  the  vegetative  cells  when  the  archicarp  is  functionless  or  absent  (Fraser 


FERTILIZATION 


291 


on  Humaria  rutilans  1907,  1908;  Carruthers  on  Helvetia  crispa  1911; 
Blackman  and  Welsford  on  Poly  stigma  rvbrum  L912  ,  P>.\  the  above 
investigators  this  early  fusion  was  regarded  as  a  sexual  one,  thai  in  the 
ascus  being  vegetative  in  nature;  and  some  described  a  "double  reduc- 
tion" in  the  ascus  to  compensate  for  the  two  nuclear  fusions. 
p.  223.) 

In  a  series  of  somewhat  later  researches  another  group  of  observers 
found  the  evidence  for  an  early  fusion  to  be  very  unsatisfactory,  and 
concluded  that  the  only  nuclear  union  in  the  life  cycle  is  thai  occurring 
in  the  ascus:  with  Dangeard  they  saw  in  this  union  the  sexual  act.  Fur- 
thermore, no  "double  reduction''  was  found  in  the  ascm.  Among  the 
researches  supporting  this  view,  which  now  appeals  to  be  the  more 
probable,  may  be  cited  the  following:  Claussen  on  Pyroncma  confliu 


Fig.  116. 

A,  nuclear  fusion  in  the  ascus  of  Peziza  vesiculosa. 
fusion  in  aeciospore  sorus  of  Phragmidium  speciosinn. 


- 


B 


{After  Dangeard,   1894 
AfU  r  Christman,  L90S 


B.  cell 


1907,  1912;  Schikorra  on  Monascus  1909;  \V.  H.  Brown  on  Pyronema 
confluens  1909,  Lachnea  scutellata  1911,  and  Leotia  1910;  Faull  on  Lab- 
oulbenia  1911,  1912;  Blackman  on  ColUma  pulposum  1913;  Nienburg  on 
Polystigma  rubrum  1914;  Ramlow  on  Ascophanus  cdrneus  and  Ascobolus 
immersus  1914;  Brooks  on  Gnomon  in  erythrostoma  1910;  McCubbin  <>n 
Helvetia  elastica  1910;  H.  B.  Brown  on  Xylaria  terUaculaia  L913;  and  Fitz- 
patrick  on  Rhizina  undulata  1918'/. 

As  the  two  nuclei  fuse  in  the  young  ascus  Harper  I  L905  observed  in 
the  case  of  Phyllactinia  corylea  that  not  only  the  chromatin  Bystems  but 
also  the  nucleoli  and  "central  bodies'  centrosomes  upon  which  the 
chromatin  strands  converge,  unite  In  the  Ascomycetes  generally  the 
fusion  nucleus,  or  "primary  ascus  aucleus,'  undergo*  -  three  successive 
mitoses  to  form  the  eight  ascospore  nuclei,  the  spore  walls  in  each  case 
being  formed  in  association  with  the  curving  astral  rays  which  focus  upon 
the  centrosome.     (See  p.  80.) 


292  INTRODUCTION  TO  CYTOLOGY 

In  certain  yeasts  it  has  been  shown  (see  Guilliermond  1920)  that  the 
production  of  ascospores  is  preceded  by  a  copulation  of  two  cells  with  a 
fusion  of  their  nuclei,  the  fusion  nucleus  dividing  to  form  the  spore  nuclei. 
A  somewhat  similar  copulation  of  the  ascospores  themselves  has  also 
been  observed  in  a  few  cases. 

Among  the  Basidiomycetes  the  nuclear  phenomena  are  best  known 
in  the  case  of  the  rusts,  owing  to  the  researches  of  Blackman  (1904), 
Christman  (1905),  and  a  number  of  later  writers.  In  the  typical  rust 
life  cycle  there  is  a  fusion  of  uninucleate  cells  at  the  base  of  the  aecial 
sorus  (Fig.  116,  B).  The  binucleate  cells  thus  arising  produce  the  binu- 
cleate  aeciospores;  and  these  upon  germination  form  a  mycelium  with 
binucleate  cells,  the  two  nuclei  dividing  in  unison  ("conjugately")  at 
each  cell-division.  After  producing  a  series  of  crops  of  binucleate 
uredospores  this  mycelium  eventually  bears  teliospores  which  may  con- 
sist of  one  or  more  cells.  In  each  cell  of  the  teliospore  the  two  nuclei 
delivered  to  it  as  the  result  of  the  conjugate  divisions  throughout  the 
binucleate  mycelium  finally  unite,  initiating  the  uninucleate  phase  of 
the  life  cycle.  Here  the  fusion  of  sexual  cells  and  the  fusion  of  their 
nnclei — two  events  which  in  most  organisms  occur  very  near  each  other 
in  time — are  widely  separated  in  the  cycle.  The  two  nuclei  dividing 
conjugately  constitute  together  a  synkaryon  in  many  respects  equivaleng 
to  a  diploid  nucleus.  Since  there  is  as  yet  no  evidence  to  show  in  what 
degree  the  two  effects  of  fertilization  (the  stimulus  to  development  and  the 
mixing  of  hereditary  lines)  are  brought  about  in  the  rusts  by  the  fusion 
of  the  sexual  cells  on  the  one  hand  and  by  the  final  union  of  their  nuclei 
on  the  other,  it  seems  best  to  regard  the  two  fusions  as  two  phases  of  the 
fertilization  process  in  spite  of  their  wide  separation  in  the  life  history. 

In  the  Hymenomycetes  it  has  been  known  for  some  time  that  a  fusion 
of  two  nuclei  occurs  in  the  basidium,  itself  the  terminal  cell  of  a  binucleate 
hypha,  prior  to  the  formation  of  the  four  basidiospore  nuclei  (Fig.  79). 
The  origin  of  the  binucleate  condition  in  the  mycelium  which  has  ap- 
parently arisen  from  a  uninucleate  spore  has  long  been  an  obscure  point. 
It  has  recently  been  shown  by  Miss  Bensaude  (1918)  in  the  case  of 
Coprinus  fimetarius  that  the  binucleate  hyphse  arise  as  the  result  of  cell 
fusions  ("plasmogamy;"  "pseudogamy")  between  uninucleate  hyphse 
arising  from  different  spores,  and  that  no  carpophores  are  produced  upon 
a  uninucleate  mycelium  arising  from  a  single  spore.  Thus  it  appears 
that  in  at  least  some  hymenomycetes  the  sexual  process  is  initiated  by  a 
fusion  of  two  cells  of  different  strains  ("plus"  and  "minus"),  as  in  the 
heterothallic  molds. 

Bryophytes  and  Pteridophytes. — In  bryophytes  and  pteridophytes 
the  details  of  the  union  of  the  motile  spermatozoid  with  the  egg  in  the 
archegonium  have  been  described  in  very  few  cases.  In  the  former 
group  may  be  cited  the  works  of  Garber  (1904)  and  Black  (1913)  on 


FERTILIZATION 


Riccia,   Meyer   (1911)   on  Corsinia,   Graham    (1918)    on   Pn  and 

Woodburn  (1920)  on  Reboulia.    It  appears  thai  in  bryophytea  the  body 

of  the  biciliate  spermatozoid,  which  consists  mainly  of  nuclear  material, 
undergoes  in  the  egg  cytoplasm  a  1  ransformal ion  into  ,-i  pel  iculate  nucleus 
before  fusing  with  the  egg  nucleus  (Fig.  117).  The  fat-'  of  the  non- 
nuclear  structures  (cytoplasm,  blepharoplast,  and  filial  i<  not  known 
with  certainty,  but  it  is  probable  that  they  arc  absorbed  in  Up 
plasm.  In  the  liverwort,  Preissia 
quadrata,  Miss  Graham  has  found 
two  centrosomes  with  weakly  de- 
veloped asters  in  the  cytoplasm  of 
the  egg  at  the  time  the  two  pronuclei 
are  about  to  fuse  (Fig.  23,  A).  It  is 
not  known  what  relation  their  ap- 
pearance may  have  to  the  entrance 
of  the  spermatozoid. 

The  most  detailed  account  of  fer- 
tilization in  a  pteridophyte  is  that 
given    by    Yamanouchi    (1908)    for 


•£■•■     . ;  '-ft*     £ 


'J'-f-i 


■v. 


■ 


?  %>     -,'•-•  *^'  <^w    !r 


Fig.  117.  Fio.    118. 

Fig.  117. — Fertilization  in  Anthoceros.     Male  and  female  pronuclei  about   c>  fuse  in 
lower  part  of  egg  in  venter  of  archegonium;  elongated  plastid  above  them.     Gametopl 

cells  show  one  nucleus  and  one  plastic!  each.      X  10.50. 

Fig.  118. — Fertilization  in  Nephrod&utn. 
A,  spermatozoid  entering  egg  nucleus.     B,  spermatosoid  becoming  retioulate  in  n 

of  female  reticulum.     {After  Yamanouchi,  1908.) 

Nephr odium  (Fig.  118).  In  Nephr odium  the  multiciliate  Bpermatosoid 
enters  bodily  into  the  egg  nucleus  with  do  previous  alteration  into  the 
reticulate  state.  Here  it  gradually  becomes  reticulate  and  irregular  in 
shape,  until  finally  its  limits  are  indistinguishable,  the  chromatic  material 
contributed  by  the  two  gametes  apparently  forming  a  Bingle  fine-meshed 
network. 

Gymnosperms. — Among    living    gymnosperms    the    Cycadales    and 
Ginkgoales  are  characterized  by  the  possession  of  motile  spermatoaoids. 


294 


INTRODUCTION  TO  CYTOLOGY 


These  spermatozoids  are  very  much  alike  in  structure  and  behavior  in 
the  two  groups,  and  are  unusually  large,  being  easily  visible  to  the  naked 
eye.  The  body  is  made  up  of  a  large  nucleus  surrounded  by  a  thin 
cytoplasmic  layer  in  which  is  imbedded  a  long,  spirally  coiled  blepharo- 
plast  bearing  many  cilia  (Fig.  114,  C).  The  behavior  of  the  spermatozoid 
in  fertilization  has  been  studied  in  Ginkgo  by  Hirase  (1895,  1918)  and 
Ikeno  (1901);  in  Cycas  revoluta  by  Ikeno  (1898);  in  Zamia  floridana  by 
Webber  (1901);  and  in  Dioon  edule,  Ceratozamia  mexicana,  and  Stangeria 

paradoxa  by  Chamberlain  (1910,  1912,  1916). 
In  all  cases  the  entire  spermatozoid  penetrates 
into  the  egg  cytoplasm,  where  the  nucleus  frees 
itself  from  the  cytoplasmic  sheath  with  its 
blepharoplast  and  cilia  and  advances  alone  to 
the  egg  nucleus,  with  which  it  fuses  (Fig.  119). 
The  behavior  of  the  chromatin  during  the  fusion 
is  not  well  known  in  either  Ginkgo  or  the  cycads. 
In  the  Coniferales  and  Gnetales  the  male 
cells  have  no  motile  apparatus.  Each  consists 
of  a  nucleus  surrounded  by  a  more  or  less 
sharply  delimited  mass  of  cytoplasm.  In  most 
cases  this  cytoplasm  remains  intact  until  after 
the  male  cell  has  entered  the  egg,  but  in  other 
forms,  such  as  Pinus,  it  mingles  with  the  cyto- 
plasm of  the  pollen  tube,  so  that  only  male 
nuclei,  rather  than  completely  organized  male 
cells,  are  delivered  to  the  egg.  All  the  nuclei 
present  in  the  pollen  tube — stalk  nucleus,  tube 
nucleus,  the  two  male  nuclei,  and  in  certain 
species  free  prothallial  nuclei — may  be  dis- 
charged into  the  egg.  All  but  the  functioning 
male  nucleus  usually  degenerate  at  once,  but  in 
some  cases  they  have  been  observed  to  undergo 
division. 
When  a  complete  male  cell  enters  the  egg  the  cytoplasm  of  the  former 
shows  two  general  modes  of  behavior.  In  some  species  it  may  be  left 
behind  in  the  peripheral  region  of  the  egg  as  the  male  nucleus  frees  itself 
and  advances  alone  to  the  female  nucleus.  This  type  of  behavior  has 
been  reported  in  Pinus  (Ferguson  1901,  1904),  Thuja  (Land  1902), 
Juniperus  (Noren  1904),  Cryptomeria  (Lawson  1904),  and  Libocedrus 
(Lawson  1907).  In  Sequoia  (Lawson  1904)  the  male  nuclei  escape  from 
their  cytoplasm  before  their  discharge  from  the  pollen  tube,  and  enter 
the  egg  alone.  In  a  second  group  of  species  the  male  cytoplasm  remains 
intact  and  invests  the  fusing  sexual  nuclei,  being  clearly  distinguishable 
from  the  cytoplasm  of  the  egg.     The  pollen  tube  cytoplasm  often  plays 


Fig.  119. — Fertilization 
in  Zamia.  Male  nucleus 
uniting  with  egg  nucleus  at 
center;  cytoplasmic^sheath 
with  spiral  blepharoplast 
above.  Another  sperm 
outside  egg.  X  25.  (After 
Webber,  1901.) 


FERTlLIZMIdS 


295 


a  conspicuous  part  in  the  formation  of  this  "mantle."  This  phenomenon 
the  significance  of  which  can  only  be  conjectured,  is  found  in  Taxodium 
(Coker  1903),  Torrcya  californica  (Robertson  1901).  To,,-, yQ  taxifolia 
(Coulter  and  Land  1905),  Cephalotaxus  Fortunei  (Coker  L907  .  Ephedra 
(Berridge  and  Sanday  1907;  Land  1907),  PhyUocladus  (Kildahl  1908 
Juniperus  (Nichols  1910),  Agathis  (Eamea  L913),  and  Taxus  (Dupler 
1917). 

Chromosome  Behavior. — The  behavior  of  the  chromosomes  duri 
the  fusion  of  the  sexual  nuclei  and  the  first  embryonal  division  has 
been  described  in  a  number  of  conifers.  As  a  general  rule,  to  judge  from 
the  data  at  hand,  the  chromatin  contributions  of  the  two  pronuclei  do 
not  become  intimately  associated  in  the  fusion  nucleus,  but  remain 
distinguishable  until  the  first  embryonal  mitosis  occurs.  Each  of  the 
pronuclei  then  gives  rise  to  its  complement    of    chromosomes    which 


B 


Fig.   120 — Fertilization  in  I' inns. 

A,  male  nucleus  pressing  into  female  nucleus.      X  140.     B,  first  embryonal   mitt 
showing  separate  paternal  and  maternal  chromosome  groups.      X  472.     {A/U     /'■ 
1904.) 


become  arranged,  often  as  two  separate  groups,  upon  a  common 
spindle.  Such  an  independent  formation  of  the  male  and  female 
chromosome  groups  has  been  observed  in  Pinus  (Blackmari  L898;  (  'hani- 
berlain  1899;  Ferguson  1909,  1904)  (Fig.  120),  Larix  (Woyciki  L899 
Tsuga  candensis  (Murrill  1900),  Juniperus  communis  (Nore*n  L907  . 
Cunninghamia  (Miyake  1910),  and  Abies  (Hutchinson  L915  ,  In 
Sequoia,  on  the  other  hand,  Lawson  I  1904)  reports  thai  the  two  nuclei 
form  a  common  reticulum  in  which  the  male  and  female  constituents 
cannot  be  distinguished.  With  regard  to  the  firsl  embryonal  mitosis 
the  general  opinion  has  been  that  all  the  chromosomes,  paternal  and 
maternal,  split  longitudinally,  the  daughter  chromosomes  being  distri- 
buted to  the  daughter  nuclei  as  in  any  other  somatic  mitosis.  Tin- 
type of  behavior  was  described  for  the  chromosomes  of  Pinus  by  M 
Ferguson  (1904)  and  at  once  came  to  be  regarded  as  general  tor  .(.in- 
fers, as  it  had  been  for  other  organisms. 

A  new  interpretation  differing  in  certain  fundamental  point.-  from  the 
above  has  been  more  recently  suggested  by  Hutchinson  1 1915  .  as  B  n  suit 


296 


INTRODUCTION  TO  CYTOLOGY 


of  his  work  on  Abies  balsamea.  According  to  Hutchinson  (Fig.  121) 
there  appear  in  the  fusion  nucleus  two  groups  of  chromosomes,  each 
containing  the  haploid  number  (16).  A  spindle  is  differentiated  about 
each  group;  and  the  two  spindles  soon  unite  to  form  one,  thus  bringing 
the  two  chromosome  groups,  representing  the  two  parental  contributions, 
into  closer  association.  The  chromosomes  now  approximate  two  by  two 
to  form  16  pairs.  The  members  of  each  pair  twist  about  each  other  and 
become  looped;  each  of  them  becomes  transversely  segmented  at  the 
apex  of  the  loop,  forming  32  (2x)  pairs  of  segments;  these  pairs  separate 
to  form  64  (4x)  chromosomes;  a  new  spindle  is  formed  and  32  (2x) 
chromosomes  pass  to  each  pole. 


ABk  £5 


I*  •  •«•) 


FUSION 

NVCklUt 


TWISTING     I  OOPinC. 


SttMfcltTflTION 


HX    StontNTS 


fIRST   TWO 
CnSBTO    NUCLII 

(i*  •  it.) 


If.   TO   torn    TOLl 


jTIG<   12i. — The  behavior  of  the  chromosomes  in  fertilization  and  the  first  embryonal  mitosis 

in  Abies,  according  to  Hutchinson.      (1915.) 

This  interpretation  of  chromosome  behavior  at  fertilization  is  remark- 
able not  only  because  it  indicates  features  resembling  those  of  the  hetero- 
typic prophase,  but  chiefly  because  it  actually  calls  for  a  qualitative 
reduction  of  the  chromatin  at  the  first  embryonal  mitosis  if  the  chroma- 
tin is  not  qualitatively  the  same  throughout  the  nucleus.  This  impli- 
cation has  not  been  discussed  by  the  advocates  of  the  new  theory.  The 
chromosomes  pair  and  twist  about  one  another  in  a  way  that  parallels 
closely  their  behavior  during  the  prophase  of  a  reduction  division.  That 
the  doubleness  seen  is  due  to  a  pairing  and  not  to  a  splitting  as  has 
heretofore  been  held  is  supported  by  the  assertion  that  the  pairs  are 
present  in  the  haploid  number,  rather  than  in  the  diploid  number  as 
would  be  the  case  were  a  splitting  of  all  the  chromosomes  occurring.  If 
the  two  members  of  each  pair  were  to  separate  at  the  first  embryonal 


FERTILIZATION 


29' 


mitosis,  a  reduction,  qualitative  as  well  as  Dumerical,  in  all   respe< 
similar  to  that  accomplished  in  the  regular  heterotypic  mitosis,  would 
be  brought  about  if  the  pairing  members  are  qualitatively  different. 
But  instead  of  such  a  separation,  each  member  of  each  pail 
transversely,  giving  4x  segments  which  are  equally  distributed  to  the  two 
daughter  nuclei,  each  of  the  latter  receiving  the  diploid  number.     Since 
the  4x  segments  become  more  or  less  intermingled  before  their  distri- 
bution it  is  probably  impossible  to  determine  jusl   which  ones  pass  to 
each  pole.     If  both  halves  of  one  transversely  divided  chromosome  p 
to  one  pole  (see  Fig.  122),  that  daughter  nucleus  only,  and  not  the  oth< 
will  receive  the  kind  of  chromatin  carried  by  thai  chromosome,  so  that 


CLf.AVA&£     MITOSIS     LQUATlONAL 


A- 


0  J& 
CD  & 


FUSION   NUCltUi 


V 

St&fJLNTATlON 


V     W 


CLCAVA&C.    MlTOJli     DlffCRCNTlAl 


Fig.   122. — Diagram  showing  the  behavior  of  the  chromosomes  in  fertilisation  and  tin- 
first  embryonal  mitosis  as  usually  interpreted  (upper  part)  and  according  to  Hut.  Inn- 
interpretation  (lower  part). 


the  two  nuclei  will  be  qualitatively  different.  A  qualitative  reduction 
will  have  occurred,  but  without  a  change  in  the  Dumber  of  chromoson* 
since  each  old  chromosome  has  become  two  new  oni  It.  on  the  other 
hand,  the  two  halves  of  the  transversely  segmented  chromosome  regularly 
pass  to  opposite  poles,  each  daughter  oucleua  will  receive  a  hah'  of  each 
and  every  parental  chromosome:  thus  if  there  are  just  as  many  kinds  of 
chromatin  as  there  are  chromosomes,  these  nuclei  will  I..-  qualitatively 
alike,  just  as  they  would  be  had  the  division  hern  longitudinal  instead  of 
transverse.  But,  as  has  been  stated  in  the  chapter  on  reduction  and  will 
be  developed  at  greater  Length  in  Chapter  XVII,  there  is  a  considerable 
body  of  evidence  which  indicates  that  each  chromosome  is  Dot  only 
qualitatively  different  from  its  fellows,  but   possesses  a  linear  differen- 


298  INTRODUCTION  TO  CYTOLOGY 

tiation  of  some  sort;  so  that  the  separation  of  the  two  halves  of  a  trans- 
versely divided  chromosme  would  constitute  a  qualitative  reduction. 
If  such  actually  is  the  condition  of  the  chromatin,  and  if  the  chromosomes 
do  behave  as  Hutchinson  supposes,  a  qualitative  reduction  must  immedi- 
ately follow  each  fertilization,  and  half  of  the  resulting  body  cells  must 
have  a  constitution  differing  from  that  of  the  other  half.  Since  there  are 
known  no  chromosome  fusions  in  which  a  restoration  in  the  number  of 
qualities  is  known  to  occur,  the  number  of  these  qualities  in  a  single 
chromosome  would  in  a  few  generations  be  reduced  to  one:  in  view  of 
the  large  number  of  past  generations  this  must  have  already  occurred. 

This  new  interpretation  of  chromosome  behavior  at  fertilization  and 
the  ensuing  mitosis  is  thus  seen  to  offer  a  direct  challenge  to  those 
theories  of  heredity  that  are  based  upon  the  idea  of  chromosomes  carry- 
ing linear  series  of  differentiated  units.  It  has  now  been  put  forward  by 
Hutchinson  (1915)  for  Abies  balsamea,  by  Chamberlain  (1916)  for 
Stangeria  paradoxa,  and  by  Miss  Weniger  (1918)  for  Lilium  philade.1- 
phicum  and  L.  longiflorum.  Consequently  several  investigators  have 
renewed  the  study  of  fertilization,  and  evidence  contradictory  to  the 
new  theory  has  been  found  by  Miss  Nothnagel  (1918)  and  Sax  (1918), 
whose  researches  are  summarized  in  the  following  section  on  the 
angiosperms. 

Angiosperms. — The  angiosperms  are  characterized  by  the  occurrence 
of  "  double  fertilization,"  a  phenomenon  discovered  independently  by 
Nawaschin  (1898)  and  Guignard  (1899).  One  of  the  two  male  nuclei 
formed  by  the  male  gametophyte  and  brought  into  the  embryo  sac  by 
the  pollen  tube,  enters  the  egg  and  fuses  with  its  nucleus,  thus  forming 
the  primary  nucleus  of  the  embryo,  while  the  other  male  nucleus  fuses 
with  the  two  polar  nuclei  to  form  the  primary  endosperm  nucleus  (Fig. 
123,  B).  As  the  male  nuclei  pass  down  the  pollen  tube  they  are  usually 
unaccompanied  by  any  specially  differentiated  cytoplasm:  the  male 
gametes  are  naked  nuclei  and  not  complete  cells.  In  some  cases,  how- 
ever, male  cells  have  been  reported  (Fig.  123,  A).  When  they  are 
liberated  in  the  embryo  sac  by  the  rupture  of  the  end  of  the  pollen  tube 
any  such  cytoplasm  is  indistinguishable  from  that  of  the  sac  and  that 
discharged  from  the  pollen  tube.  The  male  nuclei  may  appear  in  all 
respects  similar  to  other  nuclei,  or  they  may  be  distinctly  vermiform, 
as  was  observed  by  Mottier  (1898)  and  later  by  many  other  workers 
(Fig.  123,  E).  That  such  vermiform  nuclei  have  the  power  of  inde- 
pendent movement  has  been  held  by  Nawaschin  (1899,  1900,  1909, 
1910)  for  Lilium  and  Fritillaria,  by  Guignard  (1900)  for  Tulipa,  and  by 
Blackman  and  Welsford  (1913)  and  Miss  Welsford  (1914)  for  Lilium 
Martagon  and  L.  auratum.  The  vermiform  condition  may  persist  until 
the  time  of  fusion,  but  in  other  cases,  such  as  Fritillaria  (Sax  1916),  it 
gives  way  to  the  ordinary  shape.     This  change  may  occur  more  rapidly 


FERTILIZATION 


in  one  male  nucleus  than  in  the  other,  bo  thai  the  two  ma}  appear  quite 
unlike  during  the  later  stages.     Miss  Welsford  also  in  the  male 

cytoplasm  certain  granules  which  she  thinks  may  represent  the  vestig 
of  blepharoplasts. 


I 


<f 


r, 
C 


/• 


F 


(V 


Fig.   123. — Fertilization  in  angiosperms. 
A,  end  of  pollen  tube  from  basal  portion  of  style  of  LUium  auratum,  showing  two  male 
cells  and  tube  nucleus.      X  250.     (After  Welsford,  1914.)      B,  double  fertilisation  in  / 
canadense:  male  and  female  nuclei  about  to  fuse  in  egg;  second  male  and  two  polar  nuclei 
fusing  at  center  of  embryo  sac;  s,  synergids,  one  degenerated;  a,   antipodalfl 
C,  fusion  of  sexual  nuclei  in  egg  of  LUium  philadelphicum.      X  1000.     I   [ft*      R 
1918.)     D,  the  second  male  and  two  polar  nuclei  in  LUium   Martagon.      X  ~ 
Nothnagel,  1918.)     E,  vermiform  male  nucleus  in  contacl    with  egg  nucleus  in    I 
durum.      X  600.     (After  Sax,  1918.)     F,  spireme  stage  of  triple  fusion  nucleus 
durum,  showing  distinctness  of  three  chromatin  contributions.      X  750. 
G,  inclusion  of  cytoplasm  in  fusing  sexual  nuclei  of  Peperomia  I 
1910.) 


Fusion  in  Egg. — As  already  staled,  one  male  nucleus  passes  into 
the  egg  and  fuses  with  the  egg  nucleus.  So  far  as  observations  enable 
one  to  say,  only  the  male  nucleus,  and  no  cytoplasm,  enters  the  egg,  a 
point  of  much  importance  in  connection  with  the  transmission  of  heredi- 
tary characters  from  the  male  parent.  It  would  be  a  matter  of  extreme 
difficulty,  however,  to  demonstrate  conclusively  thai  in  passing  through 
the  egg  membrane  the  male  nucleus  is  absolutely  freed  of  all  adherii 
cytoplasm  or  chondriosomes;  and  it  must  be  admitted  that  such  a 
demonstration  has  not  yet  been  given  in  any  case.  The  fusion  of  the  two 
sexual  nuclei  probably  occurs  in  most  cases  very  soon  after  they  come 
in  contact,  though  in  certain  forms  the  actual  fusion  is  known  to  be 


300  '     INTRODUCTION  TO  CYTOLOGY 

considerably  delayed.  The  chromatin  of  the  two  nuclei  at  the  time 
these  unite  may  be  in  the  reticulate  (resting)  condition,  the  male  and 
female  chromatins  being  indistinguishable  in  the  fusion  nucleus.  This 
situation  was  described  by  most  of  the  earlier  workers,  including  Stras- 
burger  (1900,  1901),  Mottier  (1904),  Nawaschin  (1898,  1899),  and  Ernst 
(1902).  It  has  also  been  reported  by  Sax  (1916)  in  his  recent  work  on 
Fritillaria.  In  other  cases,  as  early  reported  by  Guignard  (1891),  the 
chromatin  has  already  reached  the  spireme  stage  characteristic  of  the 
prophase,  the  male  and  female  elements  being  distinguishable  on  the 
spindle  in  the  ensuing  division  of  the  fertilized  egg.  Such  is  the  condi- 
tion, for  instance,  in  Calopogon  (Pace  1909),  Trillium  (Nothnagel  1918), 
and  Lilium  (Weniger  1918).  That  the  same  species  may  show  con- 
siderable variation  in  this  respect  is  indicated  by  the  situation  in 
Fritillaria,  in  which  Sax  (1916,  1918)  finds  that  fusion,  though  it  usually 
occurs  in  the  resting  stage,  sometimes  takes  place  after  the  spiremes 
have  been  developed.  Miss  Weniger  (1918)  reports  that  in  Lilium 
philadelphicum  and  L.  longiflorum  the  egg  nucleus  is  in  the  resting  con- 
dition and  the  male  nucleus  in  the  spireme  stage  at  the  time  of  union. 

Chromosome  Behavior. — With  regard  to  the  behavior  of  the  two  par- 
ental groups  of  chromosomes,  it  has  been  generally  held  that,  whatever 
their  condition  at  the  time  of  the  nuclear  union,  all  of  them,  both  pater- 
nal and  maternal,  split  longitudinally  at  the  first  division  of  the  fer- 
tilized egg,  the  daughter  chromosomes  so  formed  being  distributed  to 
the  two  resulting  nuclei,  just  as  in  all  the  subsequent  somatic  divisions. 
Recently,  however,  Miss  Weniger  (1918)  has  reported  a  condition  in 
Lilium  similar  to  that  described  by  Hutchinson  (1915)  for  Abies:  the 
maternal  and  paternal  chromosomes  form  pairs  and  divide  transversely 
into  daughter  segments  which  pass  to  the  poles.  (See  p.  296.)  With 
this  conclusion  other  recent  investigations  of  fertilization  in  angiosperms 
are  not  in  agreement.  Miss  Nothnagel  (1918)  finds  in  Trillium  no  such 
pairing  and  cross  segmentation  as  Hutchinson  and  Miss  Weniger  de- 
scribe, and  states  that  each  chromosome  splits  longitudinally  as  held 
by  Miss  Ferguson  for  Pinus  and  by  cytologists  in  general.  Sax  (1918) 
also  shows  that  each  paternal  and  maternal  chromosome  in  Fritillaria 
divides  longitudinally,  the  diploid  number  (24)  passing  to  each  pole. 
In  Trillium  he  reports  an  essentially  similar  state  of  affairs,  the  diploid 
number  here  being  about  28.  He  therefore  holds  that  the  first  mitosis 
in  the  fertilized  egg  is  like  any  other  somatic  mitosis,  and  that  no  mech- 
anism for  the  segregation  of  factors  of  inheritance,  such  as  occurs  at 
reduction,  exists  here.  The  outcome  of  this  controversy  is  awaited  with 
much  interest  because  of  its  great  theoretical  importance. 

Endosperm  Fusion. — The  fusion  of  the  second  male  nucleus  with 
the  two  polar  nuclei  of  the  embryo  sac  to  form  the  primary  endosperm 
nucleus  may  be  carried  out  in  a  variety  of  ways.     The  most  commonly 


FERTILIZATION  301 

reported  method  is  thai  by  which  the  two  polars  fuse  to  form  an  "em- 
bryo sac  nucleus''  before  the  entrance  of  the  pollen  tube,  the  male 
nucleus  later  being  added.  Ernst  (1902),  for  example,  found  this  to  I 
the  method  in  Paris  quadrifolia.  I.<ss  frequently  the  male  nucleus 
meets  and  fuses  with  the  polar  nucleus  of  the  micropylar  end  of  the  sac, 
the  other  polar  then  fusing  with  the  product.  This  is  the  method  de- 
scribed by  Nawaschin  (1898,  1899)  in  his  accounl  of  the  discover} 
double  fertilization  in  Lilium  Martagon  and  Fritillaria  ienella.  The 
simultaneous  fusion  of  all  three  nuclei  appears  to  1"'  a  common 
occurrence;  it  has  recently  been  described  in  some  detail  by  Miss  Nbth- 
nagel  (1918)  for  Trillium  and  Lilium.  Just  as  in  the  case  of  the  union 
of  the  first  male  nucleus  with  the  egg  nucleus,  the  chromatin  of  tin- 
second  male  and  two  polar  nuclei  may  be  either  in  the  reticulate  or  the 
spireme  condition  as  they  come  together.  In  Lilium  Martagon  Nbth- 
nagel  1918)  (Fig.  123,  D)  it  is  in  the  form  of  fine  strands,  intermediate 
between  the  resting  and  spireme  stages.  Although  the  three  nuclei  often 
appear  exactly  alike,  it  is  frequently  possible  to  distinguish  the  male 
from  the  polars,  not  only  by  its  shape  and  -mailer  size,  but  by  the 
condition  of  its  chromatin:  in  Lilium  longiflorum  (Weniger  1918  . 
example,  the  male  nucleus  is  in  the  spireme  stage  while  the  polar  nuclei 
are  still  in  the  resting  condition.  The  membranes  of  the  three  nuclei 
may  persist  for  some  time  after  they  come  into  intimate  contact,  and 
even  after  they  have  dissolved  the  chromatic  element-  of  the  three  con- 
stituent nuclei  may  in  many  cases  be  distinguished  it'  thi  tion  has 
been  made  in  a  favorable  plane.  When  fusion  occurs  in  the  resting 
stage  this  is  not  so  apparent,  but  when  it  occurs  in  the  spiren 
the  three  chromatic  groups  are  made  out  with  little  difficulty. 

Endosperm. — As  the  division  of  the  endosperm  nucleus  approach 
the  spiremes  of  its   three    constituent    nuclei   become  increasingly  dis- 
tinct, even  if  one  or  more  of  the  nuclei  have  fused  in  the  resting  Btaf 
Nothnagel  (1918),  Weniger  (1918),  and  Sax  (1918)  in  their  recent  Btudies 
all    report    this  condition    (Fig.  123,  F).     As  the  spiremes  are  beii 
developed  into  completed  chromosomes  all  of  them    :;\  in  number    Bplif 
longitudinally,  no  observer  reporting  such  a  pairing  as  some  have  thought 
to  occur  between  the  chromosomes  of  the  egg  and  first    male  nuclei. 
Miss  Nothnagel  describes  the  formation  of  a  tripolar  spindle  about  the 
chromosomes;  the   bipolar  condition   soon   develops   from    this.     Hon 
frequently  this  may  occur  is  not   known.     Eventually  in  any  case  the 
mitosis  proceeds  along  the  usual  lines  and  the  two  daughter  nuclei  n 
3x  chromosomes  each.     This  number  is  characteristic  of  all  the  cells  of 
the  endosperm  formed   by  the  repeated  division  of  these  nuclei.     An 
exceptional    condition    has   been    noted    by   Sax      L918     in   FriHUat 
Here  the  lower  polar  nucleus,  because  of  an  irregularity  in  the  mitosis 
giving  rise  to  it,   has  21  (2x)  chromosomes  instead  of  the  normal   12 


302  INTRODUCTION  TO  CYTOLOGY 

(x).  Consequently  the  female  parent  contributes  30  chromosomes  (24  in 
one  polar  nucleus  and  12  in  the  other)  to  the  endosperm,  while  the  male 
parent  contributes  only  12;  thus  the  endosperm  has  48  (4x)  chromo- 
somes instead  of  the  normal  36. 

Although  in  the  great  majority  of  known  examples  endosperm  is 
formed  by  the  repeated  division  of  a  triple  fusion  nucleus,  cases  are  known 
in  which  it  is  produced  by  the  polar  fusion  nucleus  (embryo  sac  nucleus) 
alone  without  the  male,  or  by  the  fusion  product  of  the  male  and  one 
polar,  or  by  one  polar  alone.  Combinations  of  these  three  methods  may 
be  found  in  the  same  embryo  sac.  The  development  of  the  endosperm 
may  be  initiated  by  the  formation  of  a  number  of  free  nuclei  which  are 
parietally  placed  and  in  later  mitoses  become  separated  by  walls,  or 
by  the  formation  of  walled  cells  from  the  start.  (See  Coulter  and  Cham- 
berlain, 1903.) 

The  term  xenia  was  applied  by  Focke  (1881)  to  the  effect  of  foreign 
pollen  on  the  endosperm  of  the  resulting  seed  in  angiosperms.  Thus  if 
maize  of  a  certain  strain  which  produces  seeds  with  white  endosperm  when 
self-pollinated,  is  pollinated  with  pollen  from  a  plant  whose  seeds  have 
red  endosperm,  the  endosperm  of  the  resulting  hybrid  seeds  is  red  like 
that  of  the  pollen  parent.  No  satisfactory  explanation  of  this  phenom- 
enon was  at  hand  until  the  discovery  of  double  fertilization  by  Nawas- 
chin  and  Guignard  in  1898-9.  It  then  became  clear  that  the  endosperm, 
which  was  formerly  supposed  to  contain  only  maternal  nuclear  material, 
may  show  endosperm  characters  of  the  parent  furnishing  the  pollen  for  the 
reason  that  the  latter  contributes  a  nucleus  to  the  primary  endosperm 
nucleus,  so  that  every  endosperm  cell  contains  some  nuclear  material 
from  the  pollen  parent. 

Normally  the  endosperm  cells  are  all  alike  in  containing  two  chromo- 
some sets  from  the  female  parent  and  one  set  from  the  male,  and  the  nor- 
mal inheritance  of  endosperm  characters  as  well  as  all  ordinary  cases  of 
xenia  can  be  understood  on  this  basis.  Mottled  or  mosaic  effects  in  the 
endosperm  of  maize  hybrids  were  attributed  by  Webber  (1900)  to  such 
abnormal  modes  of  endosperm  origin  as  were  referred  to  in  a  foregoing 
paragraph:  some  of  the  cells  may  have  been  formed  by  polar  nuclei 
of  purely  maternal  constitution  while  other  cells  were  the  result  of  the 
independent  division  of  the  second  male  nucleus.  Although  this  explana- 
tion may  fit  some  cases,  it  is  becoming  apparent  from  the  work  of  Emerson 
and  others  that  most  of  them  can  be  better  accounted  for  on  the  basis  of 
aberrant  chromosome  behavior,  such  behavior  having  been  observed  in 
certain  other  organisms. 

Additional  evidence  is  found  in  all  these  phenomena  for  the  theory 
that  the  nuclear  substance  in  some  way  represents  the  physical  basis  of 
inheritance.  The  second  male  nucleus  not  only  is  concerned  in  the 
initiation  of  the  development  of  the  endosperm,  but  xenia  shows  that  it 


FERTILIZATION  .:< 

also  transmits  parental  characters.  Here,  therefore,  as  in  th<  ual 
fusion  in  the  egg,  the  two  principal  effects  of  fertilization  may  be 
recognized. 

Bibliography  12 

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FERTILIZATION  105 

18946.     La  reproduction  sexuelie  dee  Ascom]  [bid*  4:  21   68.     figs.  1<>. 

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pis.  6,  7. 
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20 


306  INTRODUCTION  TO  CYTOLOGY 

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1899.  Praxis  und  Theorie  der  Zellen-  und  Befruchtungslehre. 

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1918.  Further  studies   on   the  fertilization   and   embryogeny   in  Ginkgo  biloba. 
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Hoyt,   W.    D.     1910.     Physiological  aspects  of  fertilization   and   hybridization   in 

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Jones,  W.  N.    1918.    On  the  nature  of  fertilization  and  sex.    New  Phytol.  17 :  167-188. 
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figs.  37. 


FERTILIZATION 

Land,   W.  J.  G.    1900.     Double  fertilization  in  Composite.     Bol    Gai   30-   j 

260.     pis.  15,   16. 
1902.     A  morphological  study  of  Thuja.     Ibid.  34:  249  259.     pis.  6  v 
1907.     Fertilization   and    embryogeny  of    Ephedra    trifurca.     [bid.   44:     \ 

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19046.     The    gametophyte,    fertilization    and    embryo   of    Crypi 

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1907.     The   gametophytcs,    fertilization    and    embryo    of    Cephaiotaxv 

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308  INTRODUCTION  TO  CYTOLOGY 

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1904.     On  fertilization  in  the  Saprolegniacea?.     Ibid.  18:  .Ml 


310  INTRODUCTION  TO  CYTOLOGY 

Wagek,  H.     1896.     On  the  structure  and  reproduction  of  Cystopus  candidus  Lev. 

Ann.  Bot.  10:  295-342.     pis.  15,16.     See  also  pp.  89-91. 
1899.     The  sexuality  of  fungi.     Ibid.  13:  575-597. 
Waldeyer,  W.     1888.     Ueber  Karyokinese  und  ihre  Beziehung  zu  den  Befrueht- 

ungsvorgangen.     Arch.    Mikr.    Anat.    32:    1-122.     figs.    14.     (Engl,    transl.   in 

Quar.  Jour.  Micr.  Sci.  30:  159-281.     pi.  14.     1889.) 
Walton,  A.  C.     1918.     The  oogenesis  and  early  embryology  of  Ascaris  canis  Werner 

Jour.  Morph.  30:  527-604.     pis.  9.     fig.  1. 
Warburg,    O.     1908.     Beobachtungen   iiber   die   Oxidationsprozesse   im    Seeigelei. 

Zeitschr.  Physiol.  Chem.  57. 

1910.  Ueber  die  Oxidationen  im  lebenden  Zellen  nach  Versuchen  am  Seeigelei. 
Ibid.  66. 

1911.  Untersuchungen  iiber  die  Oxidationsprozesse  im  Zellen.     Miinchener  Med. 
Wochenschr.  57. 

1914.     Beitrage  zur  Physiologie  der  Zelle,  inbesondere  iiber  die  Oxidationsgesch- 
windigkeit  in  Zellen.     Ergeb.  d.  Physiol.  14. 
Webber,  H.  J.     1900.     Xenia,  or  the  immediate  effect  of  pollen  in  maize.     U.  S. 
Dept.  Agr.,  Div.  Veg.  Path,  and  Physiol.,  Bull.  22:  pis.  4. 
1901.     Spermatogenesis  and  fecundation  in  Zamia.     U.  S.  Dept.  Agr.  Bur.  Pit. 
Ind.  Bull.  2.     pp.  100.     pis.  7. 
Welsford,    E.  J.     1907.     Fertilization  in  Ascobolus  furfuraceus.     New  Phytol.  6: 

156. 
1914.     The  genesis  of  the  male  nuclei  in  Lilium.     Ann.  Bot.  28:  265-270.     pis.  16, 

17. 

Weniger,  W.     1918.     Fertilization  in  Lilium.     Bot.  Gaz.  66:  259-268.     pis.  11-13. 

Wheeler,  W.  M.     1895.     The  behavior  of  the  centrosomes  in  the  fertilized  egg  of 
Myzostoma   glabrum   Leukart.     Jour.    Morph.    10:    305-311.     figs.    10. 
1897.     The  maturation,   fecundation  and  early  cleavage  of  Myzostoma  glabrum 
Leukart.     Arch.  d.  Biol.  15:   1-77.     pis.    1-3. 

Wildman,  E.  E.  1913.  The  spermatogenesis  of  Ascaris  megalocephala  with  special 
reference  to  the  two  cytoplasmic  inclusions,  the  refractive  body  and  the  "mito- 
chondria": their,  origin,  nature  and  role  in  fertilization.  Jour.  Morph.  24: 
421-457.     pis.  3. 

Wilson,  E.  B.     1900.     The  Cell  in  Development  and  Inheritance.     2d  ed. 

1901.     Experimental   studies   in   cytology.     I.    A   cytological   study   of   artificial 
parthenogenesis  in  sea  urchin  eggs.     Arch.   Entw.    12:   529-596.     pis.    11-17. 

figs.  12. 
Winge,  O.     1914.     The  pollination  and  fertilization  process  in  Humulus  lupulus  L. 

and  H.  Japonicus.     Comp.  Rend.  Trav.  Lab.  Carlsberg  11. 
Woodburn,  W.  L.     1920.     Preliminary  notes  on  the  embryology  of  Reboulia  hemis- 

phcerica.     Bull.  Torr.  Bot.  Club.  46:  461-464.     pi.  19. 
Woycicki,  Z.     1899.     (On  fertilization  in  Coniferse.)     pp.  57.     pis.  2.     (Russian.) 
Yamanouchi,  S.     1906.     The  life  history  of  Polysiphonia  violacea.     Bot.  Gaz.  42: 

401-449.     pis.  19-28. 
1908.     Spermatogenesis,    oogenesis,    and   fertilization   in    Nephrodium.     Ibid.  45: 

145-175.     pis.  6-8. 


CHAPTER  XIII 
APOGAMY,  APOSPORY,  AND  PARTHENOGENESIS 

APOGAMY  AND  APOSPORY 

The  life  cycle  in  all  bryophytes  and  vascular  plants  is  characterized 
by  a  regular  alternation  of  two  well  marked  phases  or  generations:  the 
gametophytej  which  arises  from  the  spore  and  produces  gametes;  and  the 
sporophyte,  which  arises  from  the  fusion  produel  of  two  gametes  and 
produces  spores.  In  such  a  normal  life  cycle  the  Dumber  of  chromosomes 
in  the  nuclei  is  doubled  at  the  union  of  the  gametes  and  reduced  to  the 
original  number  at  sporogenesis ;  t  he  gametophyte  is  I  herefore  t  he  haploid 
generation  and  the  sporophyte  the  diploid  generation,  their  limits  being 
marked  by  the  two  cytological  crises,  fertilization  and  reduction.  Such 
an  alternation  of  haploid  and  diploid  phases  has  been  discovered  in  the 
life  cycles  of  many  algae  and  fungi  also,  so  that  the  general  conception 
of  alternation  of  generations  has  been  extended  to  these  lower  group-. 
This,  however,  is  not  the  place  for  a  discussion  of  the  homologies  implied. 
It  should  be  added  that  gametophyte  and  sporophyte  may  arise  not  only 
from  each  other,  but  either  generation  may  also  multiply  by  vegetative 

means. 

Many  instances  in  which  the  above  typical  life  cycle  is  departed  from, 
and  in  which  the  correlation  between  the  alternation  of  two  generations 

and  periodic  changes  in  chromosome  Dumber  is  broken,  arc  now    known. 

the  conspicuous  examples  being  found  among  the  fern-  and  certain 
angios perms.  The  very  convenient  classification  of  Buch  abnormalities 
drawn  up  by  Vines  (1911)  is  given  as  the  basis  for  the  present  portion 
of  the  chapter.     All  dates  and  t he  matter  included  within  square  brack* 

have  been  added  by  the  present    author. 

"In  the  first  place,  the  sporophyte  may  be  developed  either  after  an 

abnormal  sexual  art,  or  without  any  preceding  sexual  act  a;  all,  a  con- 
dition known  as  apogamy.     In  the  second,  the  gametophyte  may  be 

developed  otherwise  than  from  ;i  po>t -meiot  ie  Bpore,  :i  condition  known 
as  apospory.1 

1  [Apogamy  in  ferns  was  discovered  by  Farlo*  in  1874.  Vpospor}  was  discovered 
in  mosses  by  Pringsheim  in  L876  and  in  ferns  by  Druer}  in  1884      I  ons 

of  these  phenomena  are  given  bj  Winkler    1908   and  Strasburgei    I •'  >.] 

i  1 


312 


INTRODUCTION  TO  CYTOLOGY 


Apogamy.-  The   cases   to   be   considered   under   this    head  may  be 
arranged  in  two  groups: 

1.  Pseudapogamy:  sexual  act  abnormal. — The  following  abnormalities 
have  been  observed: 

(a)  Fusion  of  two  female  organs:  observed  (Christman  1905)  in 
certain  Uredineae  (Caoma  nitens,  Phragmidium  speciosum, 
Uromyces  Caladii)  where  adjacent  archicarps  fuse:  male  cells 
(spermatia)  are  present  but  functionless. 
(6)  Fusion  between  nuclei  of  the  same  female  organ:  observed  in  the 
ascogonium  of  certain  ascomycetes,  Humaria  granulata  (Black- 


Fig.   124. — Apogamy  in  ferns. 

A,   nuclear  migration  in   gametophyte   cells  of  Lastrcea  pseudo-mas   var.   poly  dactyl  a. 

X  500.      {After   Fanner   and   Digby,    1907.)      B,    section    through    gametophyte,     showing 

young    sporophytic    tissue    (s)     "engrafted"    into    surrounding    gametophytic  tissue  (g). 

(After  Farmer   and   Digby.)      C,   sporophyte   arising   apogamously   from   gametophyte   in 

Pier  is  cretica:  b1,  first  leaf;  v,  stem  apex;  w,  root.      (After  de  Bary.) 


man  1906),  where  there  is  no  male  organ;  Lachnea  stercorea 
(Fraser  1907),  where  the  male  organ  (pollinodium)  is  present 
but  apparently  functionless.  [A  similar  condition  has  been 
reported  in  Ascobolus  furfuraceus  (Welsford  1907),  Aspergillus 
repens  (Dale  1909),  and  Ascophanus  carneus  (Cutting  1909).] 
(c)  Fusion  of  a  female  organ  with  an  adjacent  tissue-cell:  observed 
(Blackman  19046)  [Blackman  and  Fraser  1906]  in  the  archicarp 
of  some  Uredineae  (Phragmidium  violaceum,  Uromyces  Pow, 
Puccinia  Poarum) :  male  cells  (spermatia)  present  but  function- 
less. 


APOGAMY,  APOSPORY,  AND  PARTHENOGENESIS 


313 


id)  There  is  no  female  organ :  fusion  takes  place  bet  ween  i  wo  adjacent 
tissue-cells  of  the  gametophyte;  the  sporophyte  is  developed  from 
diploid  cells  ["grafted  tissue"]  thus  produced,  but  there  is  no 
proper  zygote  as  there  is  in  a,  &,  and  c:  observed  I  Farmer  [and 
Digby]  L907)  in  the  prothallium  of  certain  ferns  I  Lastrcea  p«<  udo- 
mas,  var.  polydactyla)  [Fig.  L24,  A]:  male  organs  (and  sometimes 
female)  present  l)in  functionless.  Another  such  case  is  thai 
of   Humaria   rutilans   (ascomycete),    in   which    nuclear   fusion 


m^AJ'n  J  *</'■■■/ 


Fig.     l-'.".. 
,1,  cell  fusion  in  the  sporangium  of  Aspidium  falcatum.      >    1950.        I         B. ^-40Z/™ 
1911  )     B,  incomplete  nuclear  division  in  sporangium  ol   Nephrodium  htrtijx  ■    \->  ■ 

(After  Steil    1919.)     C,  apogamy  and  sporophytic  budding  in  the  embryo  sac  o     I  Ua 

pastorates:  egg  developing  apogamously  below;  cell  of  aucellus  forming  an  embryo  above; 
two  polar  nuclei  and  one  synergid  nucleus  al  center.      [After  Murbeck,  IWJ 

has  been  observed  (Fraser  L908)  in  hyphse  of  the  hypothecium: 
the  asci  are  developed  from  these  hypha,  and  in  them  meiosis 

takes  place;  there  are  no  sexual  organs.  [A  similar  condition 
has  been  reported  in  Helvetia  crisp*,  (Carruthers  L911)  and 
Polystigma  rubrum  (Blackman  and  Welsford  1912).  It  has 
already  been  pointed  out  (p.  291)  that  many  students  of  the 
ascomycetes  deny  the  existence  of  a  nuclear  fusion  in  the 
archicarp  or  vegetative  cells,   holding  rather  that    the  only 


314  INTRODUCTION  TO  CYTOLOGY 

fusion  in  the  life  cycle  is  that  observed  in  the  asms,  and  that 
this  fusion  is  the  real  sexual  act.] 
[(e)  Fusion  of  two  haploid  sporocytes:  In  Aspidium  falcatum  (R.  F. 
Allen  1911)  a  haploid  sporophyte  arises  by  vegetative  apogamy 
from    a   haploid    gametophyte.     In    the   sporangium   the    16 
haploid  sporocytes  fuse  in  pairs,  producing  eight  diploid  cells 
(Fig.   125,  A).     In  these  cells  reduction  occurs,   32  haploid 
spores  resulting.] 
2.  Eu-apogamy :  no  kind  of  sexual  act. 
(a)    The  gametophyte  is  haploid : 

(a)  The  sporophyte  is  developed  from  the  unfertilized  haploid 
oosphere :  no  such  case  of  true  parthenogenesis  has  yet  been 
observed.  [Kusano  (1915)  has  observed  the  division  of 
the  haploid  nucleus  of  an  unfertilized  egg  in  a  few  excep- 
tional cases  in  the  orchid,  Gastrodia  elata.  Part henogeneti  c 
development  proceeds  no  further.  The  unfertilized  egg  of 
Fucus  has  been  made  to  begin  development  by  artificial 
means  (Overton  1913),  but  the  cytological  facts  are  not 
known  here.  Motile  gametes  of  certain  other  algae  have 
been  observed  to  develop  without  conjugation,  as  in 
Ectocarpus  tomentosus  (Kylin  1918).] 
(j8)  The  sporophyte  is  developed  vegetatively  from  the  gameto- 
phyte and  is  haploid :  observed  in  the  prothallia  of  certain 
ferns,  Lastrcea  pseudo-mas,  var.  cristata-apospora  (Farmer 
and  Digby  1907),  and  Nephrodium  molle  (Yamanouchi 
1908).  [In  the  gametophytes  of  Nephrodium  molle,  which 
has  antheridia  but  no  functional  archegonia,  Yamanouchi 
found  no  nuclear  migrations  such  as  Farmer  described 
in  Lastrcea  (see  Id) ;  but  there  was  haploid  grafted  tissue, 
from  which  a  haploid  sporophyte  developed.  In  Nephro- 
dium hirtipes  (Steil  1919)  a  haploid  sporophyte  arises  by 
vegetative  apogamy  from  a  haploid  gametophyte.  When 
there  are  eight  sporogenous  cells  in  the  sporangium  there 
is  an  incomplete  nuclear  and  cell  division  (Fig.  125,  B), 
each  nucleus  coming  to  have  the  diploid  number  of  chromo- 
somes. These  eight  diploid  cells  function  as  sporocytes 
and  produce  32  haploid  spores.  Steil  at  first  (1915) 
adopted  Allen's  interpretation  (le)  for  his  material,  but 
later  decided  that  the  phenomenon  observed  was  one  of  in- 
complete division,  and  not  one  of  fusion.  In  this  case, 
as  in  Aspidium  falcatum,  apogamy  is  offset  not  by  apospory 
but  by  an  abnormal  course  of  events  in  the  sporangium. 
In  Aspidium  falcatum  the  sporophyte  arises  as  in  the 
examples  mentioned  in  this  paragraph,  but  because  of 


APOGAMY,  APOSPORY,  AND  PARTHENOGENESIS  315 

the  presence  of  a  cell  and  nuclear  fusion  it  is  classified  under 

(b)    The  gametophyte  is  diploid  (sec  under  Apospory): 

(a)   The  sporophyte   is  developed  from  the  diploid  oosphere: 

observed  in  some  Pteridophyta,  viz.  certain  ferns  I  Farmer 
1907),  Athyrium  Filix-fcemina,  var.  clarissima,  Scolopend- 
Hum  vulgare,  var.  crispum-Drummond(Bf  and  Marsilia 
(Strasburger  1907);  also  in  some  Phanerogams,  viz., 
Compositas  (Taraxacum,  Murbeck  1904  :  .1  nti  nnaria  alpina, 
Juel  1898,  1900;  sp.  of  Hieracium,  Rosenberg  L906): 
Rosacese  (Eu-Alchemilla  sp.,  Murbeck  L901,  19043  Stras- 
burger 1905  [Fig.  125,  C]):  RanunculaceaB  {Thalictrum 
purpurascens,  Overton  1902).  [Also  in  the  lily,  Atamosco 
(Pace  1913),  and  Burmannia  (Ernst  1909).  Besides  tin- 
form  of  apogamy  ("ooapogamy"  or  "generative  apo- 
gamy") Antennaria  may  also  develop  embryos  from 
diploid  synergids  ("vegetative  apogamy")  and  from  cells 
of  the  nucellus  ("sporophytic  buddirg").  A  similar 
variety  of  embryo  origins  is  found  in  certain  other  angio- 
sperms.  In  many  eases  the  chromosome  number  in 
apogamous  species  is  about  twice  as  large  as  that  of  nearly 
related  forms  reproducing  sexually  (Rosenberg  1909).] 
(/?)  The  sporophyte  is  developed  vegetatively  from  the  gameto- 
phyte:  observed  (Farmer  [and  Digby]  1907)  in  the  tern 
Athyrium  Filix-foemina,  var.  clarissima. 

In  all  cases  enumerated  under  Eu-apogamy,  apogamy  is 
associated  with  some  form  of  apospory  except  Nephrodium 
molle,  full  details  of  which  have  not  yet  been  published. 
[It  is  possible  that  a  behavior  like  that  in  Aspidium 
falcatum  (le)  or  in  Nephrodium  hirtipes  (2a0)  may  occur 
in  Nephrodium  molle.]  Many  other  ferns  are  known  to  be 
apogamous,  but  they  are  not  included  here  because  the 
details  of  their  nuclear  structure  have  not  been  investigated. 
Apospory. — The  known  modes  of  apospory  may  be  arranged  as 
follows: 

1.  Pseudapospory:  a  spore  is  formed  but  without  meiosis,  80  that  it  is  diploid 
-observed  only  in  heterosporous  plants,  viz.  certain  species 
of  Marsilia  (e.g.  Marsilia  Drummondii)  where  the  megaspore  has  a 
diploid  nucleus  (32  chromosomes)  and  the  resulting  prothallium  and 
female  organs  are  also  diploid  (Strasburger  1907):  and  in  various 
Phanerogams,  some  Composite  {Taraxacum  and  AnU  nnaria  alpina, 
Juel  1898,  1900,  1904),  some  Rosacea  (Eu-AlckemiUa,  Strasburger 
1905),  and  occasionally  in  Thalictrum  purpurascens  (Overton  1902), 
where  the  megaspore  ([and]  embryo-sac)  is  diploid;  in  some  species 


316 


INTRODUCTION  TO  CYTOLOGY 


of  Hieradum  it  has  been  found  (Rosenberg  1900)  that  adventitious 
diploid  embryo-sacs  are  developed  in  the  nucellus:  these  plants 
are  also  apogamous.  [In  Marsilia  Drummondii,  which  Shaw 
(1897)  and  Nathansohn  (1909)  had  shown  to  be  apogamous,  Stras- 
burger  (1907)  found  that,  although  normal  reduction  occurs  in 
some  of  the  megasporocytes,  giving  spores  with  16  chromosomes, 
other  megasporocytes  undergo  two  divisions  neither  of  which  is 
reductional:  the  first  division  is  homceotypic  in  character  and  the 
second  is  an  additional  vegetative  mitosis  without  a  homologue 


Fig.   126. 
A,  gametophyte  with  antheridium  (anth.)  and  rhizoids  (r)  arising  aposporously  from 
tissue  of  sorus  in  Polystichum  angularc  var.  pulcherrimuvi;  sp,  sporangia.       X  70.      (After 
Bower.)     B,   gametophyte   with  archegonia   arising  from   tip   of  pinnule  in  Pohjstichu 
X  10.      {After  Bower.) 


in . 


in  the  normal  cases.     The  resulting  spores  are  therefore  diploid, 
and  ooapogamy  follows.] . 
2    Eu-apospory:    no    spore  is  formed— of  this  there  are   two  varieties: 
(a)    With  meiosis:  this  occurs  in  some  Thallophyta  which  form  no 
spores;  the  sporophyte  of  the  Fucaceae  bears  no  spores,  con- 
sequently meiosis  takes  place  in  the  developing  sexual  organs. 
The    Conjugate    Green    Algae   also    have    no    spores,    meiosis 
taking   place   in    the   germinating   zygospore   which    develops 
directly  into  the  sexual  plant. 


APOGAMY,  APOSPORY,  AND  PARTHENOGENESIS  317 

(b)  Withoul  meiosis:  the  gametophyte  is  developed  upon  the  sporo- 
phyte  by  budding;  thai  is,  spore-reproduction  is  replaced  by  a 
vegetative  process:  for  instance,  in  mosses  it  has  been  found 
possible  to  induce  the  development  of  protonema,  the  first  stage 
of  the  gametophyte,  from  tissue  cells  of  the  sporogonium: 
[In  this  way  El.  and  fan.  Marchal  (1909.  1912)  were  able  to 
product^  in  Mnium,  Bryum,  Phascunij  and  Amblystegium 
diploid  gametophytes;  these  in  turn  produced  tetraploid 
sporophytes  which  bore  diploid  spores.  In  one  case  (Ambly- 
stegium)  a  tetraploid  gametophyte  was  regenerated  from 
cells  of  the  tetraploid  sporophyte.]  Similarly,  in  certain  ferns 
(varieties  of  Athyrium  Filix-fcemina,  Scolopendrium  vulgare, 
Lastrcea  pseudo-mas,  Polystichum  angulare,  and  in  the  species 
Pteris  aquilina  and  Asplenium  dimorphum),  the  gametophyte 
(prothallium)  is  developed  by  budding  of  the  leaf  of  the  sporo- 
phyte [commonly  from  the  margin  of  the  leaf  or  from  the  tissue 
of  the  sorus  (Fig.  126)],  and  in  some  of  these  eases  it  has  been 
ascertained  that  the  gametophyte  so  developed  has  the  same 
number  (2x)  of  chromosomes  in  its  nuclei  as  the  sporophyte 
that  bears  it — that  is,  it  is  diploid. 

Apospory  has  been  found  to  be  associated  frequently  with 
apogamy  [in  the  life  cycle];  in  fact,  in  the  absence  of  meiosis, 
this  association  would  appear  to  be  inevitable." 

PARTHENOGENESIS  IN  ANIMALS1 

The  natural  development  of  an  egg  without  having  been  fertilized  by 
a  male  gamete  is  a  phenomenon  which  is  apparently  of  much  more 
frequent  occurrence  in  animals  than  in  plants.  The  best  known  examples 
are  found  among  the  rotifers,  crustaceans,  and  insects,  parthenogenesis 
being  the  regular  mode  of  reproduction  in  some  species.  Other  modes 
also  usually  occur  in  such  organisms  under  certain  conditions  or  alter  a 
certain  number  of  generations.  Parthenogenesis  is  reported  in  some 
protozoa  (Plasmodium  vivax,  Schaudinn  1902),  where  the  macrogamete, 
after  certain  nuclear  changes,  continues  the  life  cycle  without  fusing 
with  a  microgamete.  Moreover,  as  has  already  been  described  in  the 
preceding  chapter,  parthenogenesis  may  be  artificially  induced  in  the 
eggs  of  other  animal  groups,  notably  echinoderms,  mollusks,  and  amphi- 
bians, and  around  this  fact  centers  much  of  the  significant  work  of  modern 
experimental  biology.  In  commenting  upon  parthenogenetic  develop- 
ment Minchin  (1912.  p.  137)  points  out  that  "...  the  gamete  which  has 
this  power  is  always  the  female;  but  this  limitation  receives  an  explanation 
from   the   extreme   reduction   of   the   body   of   the   male  gamete   and    its 

1  The  cytological  results  of  researches  on  maturation  and  development  in  cases  of 
parthenogenesis  have  recently  been  summarized  by  Paula  Hertwig  (1920). 


318  INTRODUCTION  TO  CYTOLOGY 

feeble  trophic  powers,  rendering  \\  quite  unfitted  for  independeni  repro- 
duction, rather  than  from  any  inherent  difference  hot  ween  the  two 
sexes  in  relation  to  reproductive  activity." 

Many  normally  parthenogenetic  animal  eggs  are  known  to  have  the 
diploid  chromosome  number  as  the  result  of  a  failure  of  reduction,  a 
condition  paralleling  that  known  as  ooapogamy  in  plants.  On  the 
contrary,  there  are  some  which,  unlike  any  known  vascular  plant,  are 
haploid,  reduction  having  taken  place  in  the  normal  fashion.  Partheno- 
genesis is  often  associated  with  certain  irregularities  in  the  behavior 
of  the  polar  bodies,  as  will  be  noted  in  the  following  descriptions  of  some 
well  known  examples.  In  the  majority  of  recorded  cases  the  partheno- 
genetic egg  produces  but  one  polar  body;  in  some,  however,  two  are 
formed  as  in  all  zygogenetic  eggs  (those  developing  after  having  been 
fertilized). 

It  was  long  ago  noticed  by  Blochmann  (1888;  see  Wilson  1900,  pp. 
281-4)  that  in  Aphis  both  zygogenetic  and  parthenogenetic  eggs  are 
produced;  the  former  produce  the  usual  two  polar  bodies  while  the 
latter  have  but  one.  It  was  also  seen  that  the  polar  bodies  are  not 
budded  off  as  separate  cells,  but  remain  within  the  membrane  of  the 
egg.  Weismann  (1886,  1887),  working  on  rotifers,  concluded  that  the 
second  polar  body  has  something  to  do  with  parthenogenetic  develop- 
ment; and  Boveri  (1887d,  1890),  who  had  seen  the  chromosomes  of  the 
second  polar  body  transform  themselves  into  a  nucleus  in  the  egg  of 
Ascaris,  made  the  suggestion  that  this  second  polar  body  might  unite 
with  the  egg  nucleus  and  so  initiate  development.  Brauer  (1894)  an- 
nounced that  this  is  precisely  what  occurs  in  Artemia,  a  phyllopod 
crustacean.  In  this  organism  two  types  of  parthenogenesis  are  found. 
In  some  cases  the  nucleus  of  the  second  polar  body,  with  84  chromosomes, 
actually  does  unite  with  the  egg  nucleus,  likewise  with  84,  causing 
'fertilization"  and  the  resulting  development  of  an  individual  with  the 
diploid  number  (168)  of  chromosomes.  In  other  cases  only  one  polar 
body  is  produced,  but  reduction  is  accomplished  in  the  division  forming 
it,  and  the  resulting  haploid  egg  develops  parthenogenetically  into  an 
individual  with  only  84  chromosomes. 

In  Phylloxera  carycecaulis  (Morgan  1906,  1908,  1909,  1910,  1915) 
only  one  polar  body  appears,  but  here  no  reduction  occurs:  the  diploid 
egg  develops  parthenogenetically.  In  Nematus  lacteus  (Doncaster  1906) 
t  wo  polar  bodies  are  produced,  but  reduction  fails  and  the  diploid  egg 
proceeds  to  develop  as  in  Phylloxera. 

It  has  long  been  known  that  the  eggs  of  the  honey  bee,  Apis  mellifica, 
will  develop  either  zygogenetically  into  females  or  parthenogenetically 
into  males.  It  has  been  shown  in  both  cases  that  there  are  two  polar 
bodies  (Blochmann)  and  that  a  normal  reduction  in  the  number  of 
chromosomes    occurs    (Nachtsheim    1912,    1913).     The   fertilized    eggs 


APOGAMY,  AFOSPORY,  AND  PARTHENOGEh  ESIS  3  1  9 

develop  into  workers  or  into  queens  with  the  diploid  number  (32)  of 
chromosomes;  those  not  fertilized  develop  into  drones  with  flic  haploid 
number  (16).  (At  the  time  of  spermatogenesis  in  the  drone  no  further 
reduction  in  chromosome  number  occurs:  the  spermatozoa  retain  the 
number  present  in  the  body  cells  (16).) 

In  the  gall-fly,  Neuroterus  lenticular  is,  Doncaster  (1910-1911)  has 
shown  that  there  are  two  classes  of  parthenogenetic  females.  The  egg  of 
the  first  class  gives  off  no  polar  bodies,  retains  the  diploid  number  (20) 
of  chromosomes,  and  develops  parthenogenetically  into  a  sexual  female 
The  egg  of  the  second  class  gives  off  two  polar  bodies,  retains  the  reduced 
number  (10)  of  chromosomes,  and  develops  parthenogenetically  into 
a  male.  (The  offspring  of  the  sexual  females  and  males  constitute  the 
next  generation  of  parthenogenetic  females.) 

There  are  thus  several  organisms  in  which  both  zygogenetie  and 
parthenogenetic  eggs  are  produced.  In  some  of  them,  such  as  the  bee, 
in  which  the  same  egg  can  develop  in  either  way,  the  two  classes  of  eggs 
show  no  morphological  differences.  In  other  forms,  such  as  a  species  of 
Melanoxanthus  (a  plant  louse)  and  Sida  crystallina  (crustacean),  they 
may  differ  considerably.  The  parthenogenetic  egg,  for  example,  may 
contain  much  less  yolk  than  the  zygogenetie  one:  it  is  less  highly  differen- 
tiated, and  "still  retains  the  capacity  to  initiate  dedifferentiation  and 
reconstitution  independently  of  union  with  a  male  gamete.  In  tlii- 
respect  it  resembles  the  less  highly  specialized  cells  of  other  tissue- 
rather  than  the  gametes"  (Child  1915,  p.  408). 

It  has  recently  been  shown  that  frogs  which  have  been  induced  to 
develop  parthenogenetically  from  punctured  eggs  (Bataillon's  method)  are 
of  both  sexes  (Loeb  1921).  The  chromosome  number  in  the  females  has 
not  been  determined,  but  both  Parmenter  (1920)  and  Goldschmidt  (1920) 
report  the  diploid  number  in  males  so  derived.  The  origin  of  this  diploid 
condition  has  not  been  satisfactorily  explained.  Parmenter  suggests 
that  it  .may  be  due  to  the  retention  of  one  polar  body,  or  to  a  premature 
division  of  the  chromosomes  without  cytokinesis  just  before  the  first 
cleavage.  This  promises  to  be  an  interesting  case  in  connection  with 
the  mechanism  of  sex-determination. 

Conclusion. — To  review  the  various  theories  which  have  been  advanced 
to  account  for  the  origin  of  parthenogenesis,  its  relation  to  other  forms  of 
reproduction,  and  its  significance  in  the  life  history,  is  a  task  which  lies  be- 
yond the  scope  of  the  present  work:it  has  been  our  purposeonly  to  indicate 
some  of  the  outstanding  cytological  facts  in  certain  conspicuous  instances 
of  the  phenomenon.  The  cytological  features  have  been  accurately 
ascertained  in  only  a  very  few  cases,  and  these  show  little  agreement. 
Furthermore,  it  is  in  artificially  induced  rather  than  in  natural  partheno- 
genesis that  the  physiological  conditions  are  best  known.  In  view  of 
these  facts  it  appears  more  than  probable  that  many  more  cytological 


320  INTRODUCTION  TO  CYTOLOGY 

and    physico-chemical    data    must   be   secured   before   any   theory   ad- 
equately harmonizing  all  the  observed  phenomena  of  parthenogenesis  can 

be  formulated. 

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Kusano,    S.     1915.     Experimental   studies   in    the   embryonal   development    in    an 

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Jour.  Gen.  Physiol.  3:  539-545.  figs.  3. 
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Overton,  J.  B.     1902.     Parthenogenesis  in  Thalictrum  purpurascens.  .   Bot.  Gaz.  33: 
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«).  10.     (Bibliography.) 


CHAPTER  XIV 

THE  ROLE  OF  THE  CELL  ORGANS  IN  HEREDITY 

The  chief  interest  of  cytology  at  the  present  time  probably  lies  in  the 
relation  which  it  bears  to  the  subject  of  heredity.  From  the  time  when 
the  problems  of  cell  research  first  began  to  take  definite  shape,  especially 
since  a  connection  between  the  activities  of  the  cell  and  the  phenomena 
of  inheritance  was  suggested,  the  efforts  of  most  cytologists  have  con- 
tributed directly  or  indirectly  to  the  solution  of  two  great  and  closely 
interrelated  problems  of  biology:  the  problem  of  ontogenetic  develop- 
ment and  the  problem  of  heredity.  The  aid  which  cytology  has  afforded 
in  these  respects  has  been  invaluable.  Not  only  has  it  been  able  to 
discover  a  large  number  of  the  significant  facts  of  individual  development, 
or  ontogeny,  but  it  has  also  thrown  a  flood  of  light  upon  many  obscure 
matters  in  the  field  of  heredity,  and  has  so  come  to  be  an  important  factor 
in  the  study  of  phylogeny  and  evolution. 

The  Law  of  Genetic  Continuity. — "The  most  fundamental  contribu- 
tion of  cell-research  to  the  theory  of  heredity,"  says  Wilson  (1909),  "is 
the  law  of  genetic  continuity  by  cell-division.  Cells  arise  only  by  the 
division  of  preexisting  cells  ...  In  each  generation  the  germinal  stuff 
runs  through  the  same  series  of  transformations;  hence  that  reappearance 
of  the  same  traits  in  successive  generations  that  we  call  heredity." 

It  is  by  the  light  of  the  above  law  that  we  are  enabled  to  see  some- 
thing of  the  nature  of  the  material  continuity  which  exists  between  suc- 
cessive stages  of  the  ontogenetic  development,  and  also  between  success- 
ive generations.  It  is  to  be  remembered,  in  the  first  place,  that  all  the 
cells  of  the  adult  multicellular  organism  are  derived  by  repeated  division 
from  the  single  cell  (ordinarily  a  zygote  or  a  spore)  with  which  develop- 
ment starts,  so  that  the  causes  of  events  occurring  at  any  particular  stage 
are  to  be  sought  largely  in  the  reactions  of  cells  at  earlier  stages;  and,  in 
the  second  place,  that  the  material  link  connecting  two  successive  gen- 
erations is  a  single  organized  cell,  usually  a  gamete  or  a  spore,  which 
means  that  the  heritage  of  a  long  ancestry  is  in  some  way  represented  in 
this  single  cell  and  its  capabilities.  "The  conception  that  then1  is  an 
unbroken  continuity  of  germinal  substance  between  all  living  organisms, 
and  that  the  egg  and  the  sperm  are  endowed  with  an  inherited  organiza- 
tion of  great  complexity,  has  become  the  basis  for  all  current  theories  of 
heredity  and  development"  (Locy,  1915,  p.  224). 


324  INTRODUCTION  TO  CYTOLOGY 

Cytological  studies  have  therefore  centered  mainly  about  the  general 
organization  of  the  egg  (chiefly  that  of  animals)  as  related  to  the  character 
of  the  organism  arising  from  it  (the  problem  of  development),  and  about 
the  roles  played  by  the  various  cell  organs  of  the  gamete  in  the  transmis- 
sion of  heritable  characteristics  from  one  generation  to  the  next  (the 
problem  of  heredity).  The  character  of  the  principal  modern  theory  of 
heredity  to  which  these  studies  have  led  is  due  in  no  small  measure  to  the 
influence  of  a  number  of  earlier  hypotheses,  such  as  those  of  Darwin  and 
deVries,  and  especially  that  of  Weismann.  These  hypotheses  will  be 
reviewed  in  Chapter  XVIII,  where  their  relation  to  the  modern  cytolog- 
ical interpretation  of  heredity,  set  forth  in  this  and  the  following  three 
chapters,  will  be  discussed. 

It  is  obvious  that  an  account  of  the  physical  basis  of  heredity  would 
require  for  completeness  not  only  a  description  of  the  structural  changes 
by  which  visible  materials  are  transmitted  and  distributed  during  game- 
togenesis,  fertilization,  and  development;  but  also  a  review  of  many  phy- 
siological processes  which  accompany  these  changes,  and  through  which 
many  characters  are  brought  to  expression.  In  these  chapters  attention 
will  be  limited  largely  to  the  structural  aspects  of  the  problem. 
Among  the  physiological  changes  those  occurring  at  the  time  of  fertiliza- 
tion are  best  known,  and  have  already  been  discussed  in  Chapter  XII. 

The  R6le  of  the  Nucleus. — It  was  Ernst  Haeckel  (1866)  who  first 
advanced  the  hypothesis  that  "the  nucleus  of  the  cell  is  the  principal 
organ  of  inheritance."  Cytological  evidence  in  support  of  this  view, 
announced  by  Haeckel  as  a  speculation,  was  brought  forward  by  O. 
Hertwig  (1875  etc.),  Strasburger  (1878,  1884),  van  Beneden  (1883  etc.), 
and  a  number  of  other  investigators,  who  described  the  behavior  of  the 
nucleus  in  the  various  stages  of  the  life  cycle,  particularly  in  somatic 
cell-division,  maturation,  and  fertilization.  Two  of  these  workers,  O. 
Hertwig  and  Strasburger,  who  had  discovered  the  fusion  of  the  gamete 
nuclei  at  the  time  of  fertilization  in  animals  and  plants  respectively, 
definitely  announced  the  theory,  now  supported  by  a  considerable  body 
of  observational  and  experimental  evidence,  that  the  nucleus  is  the 
"vehicle  of  heredity."  They  held  that  hereditary  transmission  is 
through  the  nuclei  of  the  gametes,  and  that  the  chromatin  is  the  special 
inheritance  material,  or  "idioplasm,"  about  which  there  had  been  so 
much  speculation.     This  view  was  at  once  widely  adopted  by  biologists. 

The  efforts  of  many  cytologists  were  now  directed  toward  the  further 
elucidation  and  verification  of  this  nuclear  hypothesis  of  heredity,  and 
many  observations  and  experiments  apparently  demonstrated  its  essen- 
tial correctness.  It  was  noted  that,  so  far  as  could  be  discerned,  the 
spermatozoon  in  many  cases  brings  nothing  but  nuclear  material  into 
the  egg,  so  that  hereditary  transmission  from  the  male  parent  must  be 
through  the  nucleus  alone.     A  similar  condition  was  later  reported  in 


THE  ROLE  OF  THE  (ELL  ORGANS  IN  HEREDITY 


325 


plants,  Guignard,  Nawaschin  (1910),  and  Welsford  (19]  \  pointing  out 
that  in  Lilium  only  the  male  nucleus  enters  the  egg,  its  accompanying 
cytoplasm  being  rubbed  off  and  left  behind.  (Sec  p.  299.)  Certain 
ingenious  experiments  of  Boveri  (1889,  L895j  also  L909  and  1918)  led 
to  the  same  conclusion  regarding  the  nucleus.  Boveri  induced  the  fer- 
tilization of  enucleated  fragments  of  Sphcerechinus  eggs  (a  phenomenon 


Fig.   127. 

A,  egg  of    Sphcerechinus  granulans  undergoing  artificially  induced  cleavage   1 1 1  i t « .- i ~ ; 

spermatozoon  of  Strongylocentrotus  lividus  lias  entered  and  taken  the  form  of  a  chromosome 
group.  B,  cytokinesis  beginning;  one  blastomere  will  have  a  purely  maternal  nucleus, 
ami  the  other  ;i  hybrid  nucleus.     (Diagrammed  after  Herbst,  !!><)<».) 


Fig.    127    bis. — Diagram  showing   the    irregular  distribution  of  the  chromosomes   by   a 

quadripolar  mitotic  figure.     (After  Boveri.) 


known  as  merogony)  by  spermatozoa  of  Echinus,  and  obtained  larvae 
which  were  purely  paternal  in  character.  From  this  it  was  argued  thai 
it  is  the  sperm  nucleus  alone,  and  not  the  egg  cytoplasm,  that  transmits 
the  hereditary  characters  from  one  generation  to  the  next  in  tin's  case. 
Other  experiments  of  a  similar  nature,  however,  tinned  oul  differently. 
as  will  presently  he  noted.  Certain  echinoderm  hybrids,  furthermore, 
show  paternal  larval  characters  even  when  the  egg  nucleus  has  not  been 
removed. 


326  INTRODUCTION  TO  CYTOLOGY 

A  strong  piece  of  evidence  supporting  Boveri's  conclusion  was  fur- 
nished by  Herbst  (1909).     By  treating  eggs  of  Sphcerechinus  with  valeri- 
anic acid  Herbst  caused  them  to  undergo  cleavage  artificially.     While 
the  cleavage  mitosis  was  in  progress  a  spermatozoon  of  Strongylocentrotus 
was  allowed  to  enter  the  egg,  where  it  at  once  gave  rise  to  its  group  of 
chromosomes   (Fig.    127).     These,   however,   arriving   too   late   to   join 
regularly  in  the  mitosis,  were  incorporated  in  neither  of  the  daughter 
nuclei  of  the  first  cleavage :  they  resumed  the  form  of  a  nucleus,  and  this 
was   included   in   one   of  the   blastomeres.     This   blastomere   therefore 
contained  two  nuclei,  one  maternal  and  one  paternal,  which  combined 
during  subsequent  stages,  whereas  the  other  blastomere  had  a  maternal 
nucleus  only.     Herbst  regarded  such  nuclear  behavior  as  responsible  for 
the  frequently  found  larva?  which  are  hybrid  in  character  on  one  side 
and  purely  maternal  on  the  other.     This  experiment  has  been  held  to 
show  not  only  that  it  is  the  nucleus  of  the  spermatozoon  which  brings  in 
the  paternal  characters,  but  also  that  it  is  the  chromosomes  alone  that  are 
responsible.     It  is  assumed,  though  perhaps  without  sufficient  evidence, 
that  the  other  nuclear  materials  (karyolymph  etc.)  which  may  be  present, 
as  well  as  any  cytoplasmic  elements,  have  opportunity  to  mix  generally 
with  the  egg  cytoplasm,  since  the  membrane  of  the  male  nucleus  breaks 
down  and  leaves  the  chromosomes  lying  free  before  the  egg  divides  into 
the  two  blastomeres.     The  paternal  characters,  however,  appear  only 
where  the  chromosomes  come  to  be  located — that  is,  in  the  cells  compos- 
ing one-half  of  the  organism.     In  his  work  on  multipolar  mitoses  in  di- 
spermic  eggs  (Fig.  127  bis;  see  also  p.  163)  Boveri  (1902,  1907)  was  able 
to  show  further  that  abnormal  chromosome  distribution  is  associated 
with  abnormalities  in  development  in  a  very  definite  way;  and  that  if 
isolated  blastomeres  resulting  from  such  abnormal  divisions  be  made  to 
develop  independently,  completely  normal  larvae  result  only  where  there 
is  statistical  reason  to  believe  that  a  full  complement  of  the  qualitatively 
different  chromosomes  is  present. 

The  nuclear  theory  had  its  opponents  from  the  beginning.  Verworn, 
Waldeyer,  Rauber,  and  other  early  investigators  held  that  the  cytoplasm 
as  well  as  the  nucleus  must  be  concerned  in  the  hereditary  process,  since 
the  spermatozoon  in  many  cases  does  bring  cytoplasm  into  the  egg,  and 
also  because  neither  nucleus  nor  cytoplasm  can  function  independently 
of  the  other.  This  view  received  support  in  certain  experiments  which 
seemed  to  discount  the  power  of  the  nucleus  in  controlling  heredity. 
Loeb  (1903)  found  that  when  a  sea  urchin  egg  was  fertilized  by  a  starfish 
sperm  the  resulting  larva  possessed  purely  maternal  characters,  the 
sperm  nucleus  exerting  no  visible  hereditary  effect.  The  same  thing  was 
noted  by  Godlewski  (1906)  in  crosses  between  sea  urchins  and  crinoids. 
Godlewski  made  the  further  significant  observation  that  when  enucleated 
egg  fragments  of  Parcel) i mis  (sea  urchin)   were  fertilized  by  sperm   of 


THE  ROLE  OF  THE  ('ELL  ORGANS  IX  HEREDITY  327 

Antcdon  (crinoid)  the  Larvae  so  produced,  contrary  to  BoverPs  results, 
were  maternal  in  character:  they  were  like  the  mother,  which  had  pre- 
sumably contributed  cytoplasm  only,  and  not  like  the  father,  which  had 
furnished  the  nucleus.  Fertilization  by  a  spermatozoon  had  here  pro- 
duced a  developmental  stimulus  but  no  amphimixis  (the  combining  of 
hereditary  lines),  so  far  as  could  be  judged  from  the  appearance  of  the 
larvae.  Even  if  the  male  cytoplasm  were  admitted  to  have  no  heredi- 
tary role,  it  nevertheless  seemed  that  the  cytoplasm  of  the  egg  was  clearly 
so  concerned. 

In  his  work  on  sea  urchin  hybrids  Baltzer  (1910)  was  able  to  show 
why  it  is  that  some  such  larvae  are  maternal  in  character  while  others 
have  the  characters  of  both  parents.  When  an  egg  of  Strongylocentrotus 
fertilized  by  a  spermatozoon  of  Sphcerechinus  undergoes  its  first  cleavage 
division  the  paternal  chromosomes  behave  irregularly;  they  fail  to  become 
incorporated  in  the  daughter  nuclei  and  are  lost.  Those  individuals 
which  develop  far  enough  show  maternal  skeletal  characters.  In  the 
reciprocal  cross,  on  the  contrary,  all  of  the  chromosomes  behave  normally 
and  the  resulting  larvae  are  truly  hybrid  in  character.  Thus  in  the  first 
cross,  in  which  the  paternal  chromosomes  are  lost,  the  spermatozoon 
furnishes  only  a  developmental  stimulus  and  has  no  appreciable  effect 
on  the  character  of  the  new  individual;  whereas  in  the  second  cross,  in 
which  the  paternal  chromosomes  are  included  in  the  blastomere  nuclei, 
the  spermatozoon  not  only  furnishes  the  developmental  stimulus  but 
also  contributes  paternal  characters  to  the  new  individual.  This  is 
particularly  convincing  evidence  in  favor  of  the  view  that  the  chromo- 
somes are  in  some  way  responsible  for  the  development  of  parental 
characters  in  the  offspring. 

In  a  posthumous  paper  Boveri  (1918)  reported  an  additional  observa- 
tion which,  he  believed,  goes  far  toward  explaining  the  conflicting  results 
of  different  investigators.  He  found  that  egg  fragments,  and  even  whole 
eggs,  may  often  have  chromatin  in  aform  that  easily  escapes  observation, 
but  which  can  exert  its  usual  influence  on  development.  In  accordance 
with  his  earlier  observations,  enucleate  egg  fragments  of  Sphcerechinus 
fertilized  by  spermatozoa  of  Strongylocentrotus  may  develop  into  purely 
paternal  larvae.  Most  of  them,  however,  are  intermediate  in  charac- 
ter, resembling  the  maternal  parent  also  in  certain  features.  Having 
previously  (1895,  1905)  demonstrated  that  the  size  of  the  nuclei  in 
merogonic  larvae  is  proportional  to  the  number  of  chromosomes  they 
contain  (see  Chapter  IV),  Boveri  was  able  to  show  that  the  nuclei  of  the 
intermediate  larva1  are  diploid  rather  than  haploid,  so  that  it  is  clear  that 
the  supposedly  enucleate  fragments  in  such  cases  must  have  contained 
chromosomes.  It  is  probable,  Boveri  believed,  that  the  maternal  larva1 
obtained  by  Godlewski  may  be  accounted  for  in  a  similar  fashion. 

It  was  pointed  out  by  Strasburger  (1908),  who  had  come  to  believe  in 


328  INTRODUCTION  TO  CYTOLOGY 

the  complete  monopoly  of  the  nucleus  in  the  transmission  of  hereditary 
characteristics,  that  the  maternal  character  of  Godlewski's  larva?  could 
be  explained  on  the  assumption  that  the  early  developmental  stages  do 
not  require  the  expression  of  the  hereditary  capabilities  of  the  nucleus, 
but  are  dependent  more  directly  upon  mechanical  causes.  Boveri  (1903, 
1914),  as  a  result  of  his  hybridization  experiments,  strongly  emphasized 
the  view  that  the  spermatozoon  has  an  influence  upon  all  of  the  larval  char- 
acters; but  he  pointed  out  that  the  larval  stages  by  themselves  are  not 
sufficient  grounds  upon  which  to  establish  complete  conclusions  regarding 
the  respective  roles  of  nucleus  and  cytoplasm,  since  the  general  course 
of  the  early  developmental  stages  in  such  organisms  is  immediately 
dependent  to  a  very  large  extent  upon  the  general  organization  of  the  egg. 

This  brings  us  to  a  brief  consideration  of  the  "promorphology"  of 
the  highly  organized  animal  egg,  and  of  the  relation  which  exists  between 
this  organization  and  the  character  of  the  organism  developing  from  it. 
Of  the  large  amount  of  work  done  in  this  field  only  a  hint  can  be  given 
here. 

The  Promorphology  of  the  Ovum. — There  arose  very  early  two  views 
regarding  the  organization  of  the  egg  which  recall  the  older  theories  of 
preformation  and  epigenesis  (Chapter  I).  According  to  one,  most  fully 
expressed  in  W.  His's  Theory  of  Germinal  Localization  (1874;  see  Wilson 
1900,  p.  397),  the  embryo  is  prelocalized  in  the  general  cytoplasm  of  the 
egg — not  preformed  in  the  old  sense  of  Bonnet,  but  having  its  various 
parts  represented  by  substances  with  definite  relative  positions.  This 
view  found  support  in  those  cases  in  which  a  single  isolated  blastomere  of 
the  two-celled  stage  develops  into  a  half-larva  instead  of  a  complete 
smaller  larva  (Roux  on  the  frog,  1888;  Crampton  on  the  marine  gastropod, 
Ilyanassa,  1896) ;  and  especially  in  Beroe,  a  ctenophore,  which  produces  an 
incomplete  larva  even  if  a  portion  of  the  unsegmented  egg  be  removed 
(Driesch  and  Morgan,  1895). 

Opposed  to  the  above  view  was  that  which  held  the  egg  to  be  isotropic 
and  without  any  predetermination  of  embryonic  parts.  Certain  well 
known  experiments  appeared  to  bear  out  this  conclusion.  It  was  found 
in  the  frog  (Pfluger  1884;  Roux  1885),  the  sea  urchin  (Driesch  1892),  an 
annelid  (Wilson  1892),  and  Ascaris  (Boveri  1910)  that  very  abnormal  types 
of  cleavage  can  be  artificially  induced,  but  that  normal  larvae  nevertheless 
result.  A  number  of  cases  were  also  described  in  which  complete  embryos 
arose  from  single  isolated  blastomeres  of  the  two-celled  stage  (Fundulus, 
Morgan  1895;  and  other  forms),  or  even  from  those  of  the  sixteen-celled 
stage  (Clytia,  Zoja  1895).  Had  there  been  any  prelocalization  of  parts 
in  the  egg  it  is  difficult  to  see  how  normal  or  complete  embryos  could 
have  arisen  in  such  abnormal  ways  as  these. 

It  appears  that  the  eggs  of  different  animal  species  vary  greatly 
in  the  degree  and  fixity  of  their  internal  differentiation.     In  some  cases 


THE  ROLE  OF  THE  CELL  ORGANS  IN  HEREDITY 


329 


the  egg  is  virtually  isotropic,  and  through  several  succeeding  cell  gene- 
rations the  blastomeres  are  equipotential,  thai   is,  equally  capable  of 

developing  into  any  part  of  the  body  or  even  into  the  whole  of  it.  Thus 
the  embryonic  parts,  and  hence  many  of  the  individual's  characters,  are 
not  definitely  marked  out  until  a  comparatively  late  stage.  On  the 
contrary,  there  are  forms  in  which  the  axis  of  polarity  and  certain  funda- 
mental embryonic  parts  are  roughly  delimited  in  the  egg  cytoplasm  in 
such  a  way  that  an  alteration  in  the  relative  positions  of  the  egg  materials 
brings  about  a  corresponding  alteration  in  the  character  of  the  resulting 
individual.  As  an  illustration  of  such  internal  differentiation  may  !»<• 
taken  the  case  of  Styela,  an  ascidian,  described  by  Conklin  (1915  In 
the  egg  of  Styela  there  are  four  or  five  distinct  kinds  of  plasma  arranged 
in  a  definite  order  and  distributed  in  a  regular  manner  as  cleavage  pro- 
ceeds, each  kind  eventually  giving  rise  to  a  certain  portion  of  the  embryo. 


CTENOPHORE. 


TUKBELLATIIAN  ECHINODERn  A5CIDIAN 

Fig.  128. — Eggs  of  various  animals,  showing  the  patterns  assumed   by  the  materials 
which  give  rise  to  the  various  body  regions.      In  the  first    three  the  egg  has  undergone 
division,  and  the  plasmas  becoming  ectoderm,  mesoderm,  and  endoderm  are  represented 

in   clear   white,    cross-hatching,    and    parallel    ruling   respectively.      In    the   fourth   egg    two 
divisions  have  occurred,  and  several  definitely  arranged  substances  are  distinguishable 
(After  Conklin,  1915.) 

Substances  which  are  yellow,  gray,  slate-blue,  and  colorless  give  rise 
respectively  to  muscle  and  mesoderm,  nervous  system  and  notocord 
endoderm,  and  ectoderm.  "Thus  within  a  few  minutes  after  the  fertiliza- 
rion  of  the  egg,  and  before  or  immediately  after  the  first  cleavage,  the 
anterior  and  posterior,  dorsal  and  ventral,  right  and  left  poles  arc  clearly 
distinguishable,  and  the  substances  which  will  give  rise  to  ectoderm, 
endoderm,  mesoderm,  muscles,  notocord  and  nervous  system  are  plainly 
visible  in  their  characteristic  positions"  (Conklin  191"),  p.  lis  .  If 
such  eggs  arc  placed  in  a  centrifuge  the  various  substances  may  be 
made  to  assume  an  entirely  abnormal  stratified  arrangement  which 
in  turn  "may  lead  to  a  marked  dislocation  of  organs;  the  animal  may  be 
turned  inside  out,  having  the  endoderm  on  the  outside  and  its  skin  and 
ectoderm  on  the  inside,  etc."  (p.  321).  Such  a  behavior  emphasizes 
the  determinative  character  of  the  cytoplasmic  pattern  clearly  present 
in  many  eggs.  It  has  further  been  noted  that  the  eggs  of  various  animal 
phyla  are  characterized  by  distinct  patterns  in  the  arrangmeent  of  their 
visibly  different  materials  (Fig.   128).     "The  polarity,  symmetry   and 


330  INTRODUCTION  TO  CYTOLOGY 

pattern  of  a  jellyfish,  starfish,  worm,  mollusk,  insect  or  vertebrate  are 
foreshadowed  by  the  characteristic  polarity,  symmetry  and  pattern  of  the 
cytoplasm  of  the  egg  either  before  or  immediately  after  fertilization" 
(Conklin,  p.  172-5).     That  the  arrangement  of  the  embryonic  parts  is 
not  solely  dependent  upon  such  visible  egg  substances  is  shown  by  the 
observations  of  Morgan  (1909c,  1910e)  and  Boveri  (19106)  on  centrifuged 
eggs  of  Arbacia,  Ascaris,  the  frog,  and  other  forms.     Here  it  is  found 
that  the  displacement  of  the  various  substances  does  not  necessarily 
cause  a  dislocation  of  the  body  parts  of  the  embryo:  hence  the  setting 
apart  of  the  embryonic  regions  must  be  dependent  upon  a  polarity  in  the 
egg  which  at  least  in  many  cases  is  not  disturbed  by  the  experimental 
alteration  in  position  of  the  visible  egg  substances.     But  in  either  case 
differentiation   appears   to   be   related  to   a   cytoplasmic    organization. 
From  this  it  would  appear  that  the  characters  which  such  an  organism 
inherits  from  the  preceding  generation  do  not  belong  to  one  category  and 
are  not  transmitted  in  the  same  way.     There  are  first  those  general 
characteristics  of  organization  which  are  the  direct  outgrowth  of  a  corre- 
sponding organization  in  the  egg  cytoplasm.     Secondly,  there  are  the 
Mendelian  characters  which  appear  later  in  the  ontogeny  and  which  there 
is  every  reason  to  believe  are  represented  in  some  way  in  the  chromosomes 
of  the  gamete  nuclei   (Chapter  XV).     Boveri  thus  distinguished  two 
periods  after  fertilization:  an  early  one  in  which  the  course  of  develop- 
ment is  dependent  on  the  organization  of  the  egg  cytoplasm,  only  general 
metabolic  functions  of  the  chromosomes  being  active;  and  a  later  one  in 
which  the  specific  hereditary  powers  of  the  chromosomes  are  brought  to 
expression,    the   right   chromosomal    combination   then    proving   to   be 
necessary  for  normal  development. 

The  question  naturally  arises  as  to  how  much  the  cytoplasmic  organ- 
ization may  be  due  in  turn  to  the  activity  of  the  nucleus  during  the 
differentiation  of  the  egg — as  to  whether  the  general  characters  which  are 
the  direct  outgrowth  of  this  organization  may  or  may  not  be  ultimately 
dependent,  as  are  the  clearly  Mendelian  characters,  on  nuclear  factors. 
Conklin  (1915)  comments  upon  this  point  as  follows:  "In  this  differentia- 
tion and  localization  of  the  egg  cytoplasm  it  is  probable  that  certain 
influences  have  come  from  the  nucleus  of  the  egg,  and  perhaps  from  the 
egg  chromosomes.  There  is  no  doubt  that  most  of  the  differentiations  of 
the  egg  cytoplasm  have  arisen  during  the  ovarian  history  of  the  egg,  and 
as  a  result  of  the  interaction  of  nucleus  and  cytoplasm;  but  the  fact 
remains  that  at  the  time  of  fertilization  the  hereditary  potencies  of  the 
two  germ  cells  are  not  equal,  all  the  early  stages  of  development,  includ- 
ing the  polarity,  symmetry,  type  of  cleavage,  and  the  pattern,  or  relative 
positions  and  proportions  of  future  organs,  being  foreshadowed  in  the 
cytoplasm  of  the  egg  cell,  while  only  the  differentiations  of  later  develop- 
ment are  influenced  by  the  sperm.     In  short  the  egg  cytoplasm  fixes  the 


THE  ROLE  OF  THE  (ELL  ORGANS  IN  HEREDITY  331 

general  type  of  development  and  the  sperm  and  egg  nuclei  supply  only 
the  details"  (p.  L76). 

Plastid  Inheritance.  Certain  cases  of  'plastid  inheritance'1  have 
been  brought  forward  to  show  that  the  character  of  an  organism  may  not 
be  entirely  due  to  factors  delivered  to  it  by  the  gamete  or  spore  nuclei. 
It  has  been  pointed  out  that  two  successive  generations  of  cells  repro- 
ducing by  division  resemble  each  other  for  the  obvious  reason  thai  tin- 
organs  of  any  given  cell  may  actually  become  the  corresponding  organs  of 
the  daughter  cells.  Thus  in  the  case  of  a  unicellular  green  alga  the 
daughter  individuals  are  like  the  mother  individual  in  being  green  because 
the  chloroplast  of  the  mother  cell  is  divided  and  passed  on  directly  to 
them.  In  those  alga?  in  which  a  swarm  spore  germinate-  to  produce  ;i 
multicellular  individual  {Ulothrix  etc.),  or  associates  with  others  of  it- 
kind  to  form  a  colony  {Hydrodictyon,  Pediastrum;  Harper  1908,  I918a6), 
the  color  of  the  successive  colonies  or  multicellular  individuals  is  a  charac- 
ter that  is  transmitted  directly  by  the  repeated  division  of  chloroplast-. 
Thus,  as  Harper  urges,  the  nucleus  is  not  required  here  to  account  for  the 
resemblance  between  successive  generations  of  cells  or  individuals,  so  far 
as  this  character  is  concerned. 

A  similar  interpretation  has  been  placed  by  some  geneticists  upon  the 
inheritance  of  "chlorophyll  characters"  in  the  higher  plants,  the  supposi- 
tion being  that  plastids,  multiplying  only  by  division,  are  responsible 
for  the  distribution,  in  the  individual  plant  and  through  successive 
generations,  of  those  characters  which  manifest  themselves  in  tin-! 
organs.  Abnormalities  in  chlorophyll  coloring  are  accordingly  held  to  be 
due  to  an  abnormal  condition  or  behavior  of  the  chloroplast  s. 

Such  a  case  is  that  of  Mirabilis  jalapa  albomaculata,  described  by 
Correns  (1909).  In  plants  of  this  race  there  are  some  branches  with 
normal  green  leaves,  some  with  white  leaves,  and  some  with  "checkered' 
(green  and  white)  leaves.  Flowers  are  borne  on  branches  of  all  three 
types.  In  all  cases  crosses  between  unlikes  result  in  seedlings  with  the 
color  of  the  maternal  parent:  inheritance  is  strictly  mat  (anal.  For 
instance,  if  a  flower  on  a  green  branch  is  pollinated  with  pollen  from  a 
flower  on  a  white  branch  the  offspring  are  all  green.  In  the  reciprocal 
cross  the  offspring  are  all  white,  and  soon  die  because  of  the'  lack  of 
chlorophyll.  In  neither  case  does  the  pollen  affect  the  color  of  tin4 
resulting  individual.  The  explanation  offered  by  Correns  for  the  color- 
less condition  is  that  it  is  due  to  a  cytoplasmic  disease  which  destroys 
the  chloroplasts.  It  is  therefore  delivered  directly  to  the  next  generation 
in  the  egg  cytoplasm,  and  is  not  transmitted  by  the  male  parent  because 
no  male  cytoplasm  is  brought  into  the  egg  at  fertilization.  If  it  had  been 
due  to  nuclear  factors  it  would  have  been  transmitted  by  both  parents, 
since  the  nuclear  contributions  of  the  two  are  equal.  This  condition  is 
analogous  to  that  occasionally  found  in  animals,  in  which  bacteria  may 


332  INTRODUCTION  TO  CYTOLOGY 

be  carried  from  one  generation  to  the  next  in  the  egg  cytoplasm,  causing 
a  direct  inheritance  of  the  disease.  But  such  pathological  cases  are  not 
to  be  confused  with,  or  thought  to  contradict,  normal  Mendelian  heredity, 
which,  as  will  be  seen  in  the  following  chapter,  is  closely  bound  up  with 
nuclear  phenomena.  They  are  rather  to  be  regarded  as  examples  of 
repeated  reinfection. 

Results  differing  from  those  of  Correns  were  obtained  by  Baur  (1909) 
in  his  researches  on  Pelargonium  zonale  albomarginata.  This  form,  which 
is  characterized  by  white-margined  leaves,  often  has  pure  green  and  pure 
white  branches,  as  in  Mirabilis.  Crosses  either  way  between  flowers  on 
these  two  kinds  of  branches  result  in  every  case  in  mosaic  (green  and 
white)  offspring:  inheritance  is  here  not  purely  maternal  as  in  Mirabilis. 
Although  Baur  admits  for  this  case  the  possibility  of  a  Mendelian  inter- 
pretation if  a  segregation  of  factors  for  greenness  and  whiteness  in  the 
somatic  cells  be  allowed,  he  thinks  it  more  probable  that  inheritance  in 
this  instance  is  not  a  matter  of  chromosomes  and  Mendelism  at  all,  but 
is  rather  due  to  a  sorting  out  of  green  and  colorless  plastids,  themselves 
permanent  cell  organs,  in  the  somatic  cells.  In  order  to  account  for 
inheritance  through  both  the  male  and  the  female,  Baur  assumes  that 
primordia  of  plastids  are  brought  in  through  the  male  cytoplasm  as  well 
as  the  egg  cytoplasm,  a  conclusion  directly  contradictory  to  that  of 
Correns.  Ikeno  (1917),  working  on  variegated  races  of  Capsicum  annum, 
obtained  results  similar  to  those  of  Baur  on  Pelargonium,  and  concluded 
that  transmission  of  variegation  is  not  through  the  nucleus,  but  through 
plastids  contributed  by  bothjparents. 

Although  the  results  and  interpretations  of  Correns  and  Baur  are  at 
present  irreconcilable  except  on  the  basis  of  assumptions  not  warranted 
by  known  facts,  they  agree  in  the  conclusion  that  plastid  inheritance  is 
not  Mendelian,  but  is  due  rather  to  extra-nuclear  factors.  Baur  reports 
corroborative  evidence  in  Antirrhinum  (1918).  Opposed  to  this  con- 
clusion is  that  of  Lindstrom  (1918),  who  has  clearly  shown  in  the  case  of 
certain  variegated  races  of  maize  that  the  inheritance  of  characters  due 
to  unusual  plastid  behavior  is  strictly  Mendelian.  This  means  that  the 
distribution  or  degree  of  prominence  of  the  plastids,  although  these  may 
be  organs  with  their  own  individuality,  depends  upon  the  activity  of 
Mendelian  factors  in  the  chromosomes,  which  represent  the  only  known 
cell  mechanism  in  which  there  is  at  present  any  hope  of  finding  an  expla- 
nation for  the  distribution  of  Mendelian  characters  (Chapter  XV).  In 
Lindstrom's  plants  plastid  inheritance  appears  to  be  as  much  a  nuclear 
matter  as  the  inheritance  of  any  other  character  manifested  in  the  extra- 
nuclear  portion  of  the  cell. 

On  the  basis  of  the  data  at  hand  the  tentative  conclusion  seems  fully 
justified  that  all  cases  of  chlorophyll  inheritance  do  not  belong  to  one 
category.     Some  of  them  are  clearly  to  be  accounted  for  on  the  same  basis 


THE  ROLE  OF  THE  CELL  ORGANS  IN  HEREDITY  333 

with  other  Mendelian  characters,  whereas  others  appear  to  require  an 
explanation  of  another  kind.  The  results  of  work  in  progressal  Cornell 
University  on  variegated  races  of  maize  points  in  this  direction.  One 
of  the  most  interesting  problems  in  cytology  and  genetics  a1  presentis 
that  concerning  the  manner  in  which  extra-nuclear  bodies,  such  as  plas- 
tids  and  their  primordia,  may  account  for  certain  types  of  inheritance, 
and   the   extent    to   which  their  behavior  may  be    influenced  by  the 

nucleus.  .     . 

Aleurone  Inheritance.— We  may  here  refer  to  the  attempt  which  has 
been  made  to  explain  the  inheritance  of  aleurone  color  in  maize  endosperm 

on  the  basis  of  a  somatic  segregation  of  special  cell  organs  in  the  form  o\ 
granular  primordia,  which  multiply  by  fission  and  develop  into  aleurone 
bodies  of  various  tvpes  and  colors.  But  since  aleurone  and  other  endo- 
sperm characters  are  inherited  in  Mendelian  fashion,  as  shown  by  Eas1 
and  Hayes  (1911,  1915),  Collins  (1911),  and  Emerson  (1918),  and  since 
there  has  been  adduced  in  support  of  the  supposed  sort  ing  out  of  primor- 
dia no  evidence  approaching  in  cogency  that  upon  which  t  he  chromosome 
theory  has  been  built  up,  geneticists  generally  are  of  the  opinion  thai  the 
chromosomes  with  their  well  known  mechanism  of  segregation  oiler  the 
best  promise  of  an  explanation  of  the  inheritance  of  aleurone  characters, 
though  all  admit  that  other  organs  may  play  a  part  in  bringing  these 
characters  to  expression.  Furthermore,  the  case  for  the  self-perpetuity 
of  the  aleurone  grain  is  much  weakened  by  the  fact  of  their  artificial  pro- 
duction by  Thompson  (1912). 

The  theory  that  chondriosomes  are  concerned  in  heredity  has  been 

discussed  in  Chapters  VI  and  XII. 

General  Conclusions.— In  conclusion  the  statement  may  again  be 
made  that  as  genetical  researches  multiply  it  becomes  increasingly  clear 
that  the  characters  in  which  an  individual  resembles  that  from  which  it 
sprang  are  not  in  every  case  transmitted  to  it  in  the  same  manner,  rhose 
characters  which  are  inherited  according  to  Mendelian  rules,  to  anticipate 
a  conclusion  based  on  evidence  to  be  presented  in  the  nexl  chapter,  mall 
probability  owe  their  repeated  appearance  in  successive  generations  to 
"factors"  of  some  sort  which  are  transmitted  by  tin-  chromosomes  oi 
the  nucleus.  This  applies  also  to  those  characters  which,  while  Men- 
delian in  distribution,  depend  lor  their  expression  upon  the  presence 
of  other  cell  organs  (plastids)  which  may  have  an  individuality  oi  the,,- 


own 


All  or  nearly  all  of  the  hereditary  contribution  made  by  the  male 
parent  must  in"  most  organisms  be  in  the  above  form,  since  the  male 
gamete  consists  almost  exclusively  of  nuclear  material.  The  female 
gamete  or  egg,  in  addition  to  the  clearly  Mendelian  characters  repre- 
sented by  factors  in  its  nucleus,  may  at  least  in  the  case  of  many  animals 


334  INTRODUCTION  TO  CYTOLOGY 

contribute  certain  general  characters,  such  as  polarity,  symmetry,  and 
general  type  of  early  development,  which  are  the  direct  outgrowth  of  an 
elaborate  organization  present  in  the  egg  cytoplasm.  It  is  true  that  this 
organization  is  the  result  of  processes  in  which  the  nucleus  cooperates 
during  the  differentiation  of  the  egg,  and  those  who  hold  to  the  universal 
applicability  of  the  Mendelian  interpretation  would  assume  that  the 
type  of  organization  must  depend  upon  Mendelian  factors  carried  in  the 
nucleus.  However  this  may  be,  the  fact  remains  that  the  two  gametes 
at  the  time  of  fertilization  are  not  equal  in  hereditary  potency,  as 
Conklin  states.  So  far  as  the  clearly  Mendelian  characters  are  con- 
cerned, however,  all  evidence  goes  to  show  that  they  are  precisely 
equal. 

The  direct  inheritance  of  metidentical  characters,  such  as  the  above 
mentioned  green  plastid  color  in  Pediastrum,  and  the  indirect  inheritance 
of  colony  characters  in  the  same  form,  afford  other  examples  of  hereditary 
transmission  otherwise  than  through  the  nucleus.  With  respect  to 
colony  characters,  Harper  has  shown  in  a  striking  manner,  both  in  Hydro- 
dictyon  and  Pediastrum,  that  the  characteristic  form  and  type  of  organiza- 
tion assumed  by  the  colony  are  the  results  of  interactions  between  the 
form,  polarities,  adhesiveness,  surface  tension,  etc.  of  the  free-swimming 
swarm  spores  which  aggregate  to  build  it  up.  The  swarm  spore  has  an 
individual  organization  of  a  particular  type,  but  its  capabilities  show  it  to 
be  devoid  of  any  arrangement  of  its  protoplasmic  parts  corresponding 
either  to  its  future  position  in  the  colony  or  to  the  arrangement  of 
the  cells  in  the  colony  as  a  whole.  The  character  of  the  colony 
thus  depends  upon  the  interactions  of  its  component  units  and  is 
in  no  way  represented  in  any  one  of  them.  Consequently  it  is  held  by 
Harper  that  no  system  of  spatially  arranged  factors  in  a  special  germ 
plasm  is  required  to  account  for  the  regular  reappearance  of  such  cell  and 
colony  characters  in  these  organisms,  and  that  such  facts  must  be  reck- 
oned with  in  attempting  to  explain  heredity  and  development  in  terms 
of  the  cell. 

By  whatever  means  they  are  transmitted,  it  is  evident  that  most 
characters  must  be  brought  to  expression  through  the  activity  of  the  cell 
system  as  a  whole,  the  process  involving  a  long  series  of  reactions  in 
which  all  or  nearly  all  of  the  cell  constituents  play  their  parts.  At  the 
present  time  little  or  nothing  is  known  of  the  real  nature  of  the  "factor" 
or  of  the  manner  in  which  it  may  influence  the  development  of  a  character. 
In  general,  then,  we  may  say  that  the  heritage  bequeathed  by  an  indi- 
vidual to  its  offspring  is  in  most  organisms  transmitted  mainly  through  the 
nucleus,  since  it  is  very  largely  upon  this  organ  that  the  development  or 
non-development  of  particular  characters  in  the  organism  depends;  but 
also  that  the  development  of  the  characters  in  the  offspring,  however  these 


THE  ROLE  OF  THE  (ELL  ORGANS  IN   HEREDITY  335 

may  be  transmitted,  involves  all  of  the  cell  organs  as  well  as  a  complicated 

and  orderly  series  of  intercellular  reactions  and  response  These  two 
phases,  the  transmission  of  a  heritage  of  factors  and  the  develop- 
ment of  the  organism's  characters  as  the  result  of  their  influence, 
must  both  be  very  much  more  fully  known  before  eit  her  can  be  adequately 
understood. 

To  some  of  the  more  cogent  evidence  upon  which  these  general  con- 
clusions are  based  we  shall  now  turn. 

Bibliography  at  end  of  Chapter  XVII 1. 


CHAPTER  XV 
MENDELISM  AND  MUTATION 

MENDELISM 

The  classic  researches  carried  out  by  Mendel  a  half -century  ago  on 
the  hybridization  of  garden  peas  are  now  so  well  known  that  a  detailed 
description  of  them  would  be  superfluous  here.  Moreover,  since  the 
main  principles  of  Mendelism  are  illustrated  in  the  results  of  the  simplest  of 
Mendel's  experiments,  a  review  of  one  or  two  of  the  latter  will  for  our 
purposes  be  sufficient.1 

A  Typical  Case  of  Mendelian  Inheritance. — Mendel  crossed  plants  of  a 
pure  bred  race  of  tall  peas  (6  to  7  feet  in  height)  with  plants  of  a  pure 
bred  dwarf  race  (%  to  \y2  feet  in  height)  (Fig.  129).  All  the  plants 
of  the  first  hybrid  generation  (Fi)  were  tall  like -one  of  their  parents. 
When  these  tall  hybrids  were  self-fertilized  or  bred  to  one  another,  it 
was  found  that  the  second  hybrid  generation  (Fz)  comprised  individuals 
of  the  two  grandparental  types,  tall  and  dwarf,  in  the  relative  numerical 
proportion  of  3:1.  It  was  further  found  that  the  tall  individuals  of  this 
generation,  though  alike  invisible  characters,  were  unlike  in  genetic  con- 
stitution: one-third  of  them,  if  bred  for  another  generation,  produced 
nothing  but  tall  offspring,  showing  that  they  were  "pure':  for  the 
character  of  tallness;  whereas  the  other  two-thirds,  if  similarly  bred, 
produced  again  in  the  next  generation  both  tall  and  dwarf  plants  in  the 
proportion  of  3  :  1,  showing  that  they  were  hybrids  with  respect  to  tallness 
and  dwarfness.  The  dwarf  plants  of  the  second  hybrid  generation  (F2) 
produced  nothing  but  dwarfs  when  interbred;  they  were  "pure,;  for 
dwarfness.  From  these  facts  it  was  evident  that  the  plants  of  the  F2 
generation,  although  they  formed  only  two  visibly  distinct  classes,  were 
in  reality  of  three  kinds:  pure  tall  individuals,  tall  hybrids,  and  pure 
dwarfs,  in  the  relative  numerical  proportions  of  1  :2:1. 

The  explanation  offered  by  Mendel  for  these  phenomena  may  be 
briefly  stated  as  follows  (Fig.  129).  The  germ  cells  produced  by  the 
pure  tall  plant  carry  something  (now  termed  a  factor,  represented  here 

1  Detailed  accounts  of  the  many  facts  of  Mendelism  may  be  found  in  more  special 
works  on  the  subject.  See  Morgan  et  at.  1915,  Chapters  1  and  2;  Bateson  1913; 
Castle,  Coulter  et  al.  1912;  Castle  1916;  Coulter  and  Coulter  1918;  Babcock  and 
Clausen  1918,  Chapter  5;  Punnet  1919;  Darbishire  1911;  Morgan  1919a;  Thomson 
191.3;  East  and  Jones  1919. 

336 


MENDELISM  AND  Ml  TATlo.X 


337 


by  T)  which  makes  the  resulting  planl  tall.     The  germ  cells  of  the  dwarf 
plant   carry  something   (/)   causing  the  dwarf  condition.     In   the   firsl 


PARENTS 


r, 


L 


Fig.  12<). — A  typical  Mendelian  cross  between  tall  and  dwarf  peas,  showing  dominance 
of  the  tall  over  the  dwarf  condition  in  the  first  hybrid  generation  (Fi),  and  the3:l  ratio  of 
tall  plants  to  dwarfs  in  the  second  hybrid  generation  (/•' ■■>.  \t  the  right  is  shown  the 
corresponding  distribution  of  tin-   Mendelian  factors  for  tallness   (T)   and  dwarfness   <n. 

hybrid  generation  (Fi)  both  factors  arc  present,  T  coming  from  one  pa  rem 

and  /  from  the  other,  bu1  T  "dominates"  and  prevents  the  expression  of 

22 


338 


INTRODUCTION  TO  CYTOLOUY 


the  "recessive"  t,  so  that  the  plants  of  this  generation  are  all  tall.  When 
the  hybrid  (Fi)  produces  germ  cells  the  two  factors  for  tallness  and 
dwarfness  separate,  half  of  the  germ  cells  receiving  T  and  the  other  half 
receiving  t.  Each  gamete  therefore  carries  either  one  or  the  other  of  the 
two  factors  in  question,  but  never  both:  a  given  gamete  is  "pure,! 
either  for  T  or  for  t.  This  segregation  in  the  germ  cells  of  factors  pre- 
viously associated  in  the  individual  without  their  having  been  altered  by 
this  association  is  the  central  feature  of  the  entire  series  of  Mendelian 
phenomena,  and  is  often  referred  to  as  Mendel's  first  law.  Since,  now, 
the  gametes,  both  male  and  female,  produced  by  the  hybrid  plants  of  the 
F \  generation  are  of  two  kinds  (half  of  them  bearing  T  and  half  bearing  t) 


M1RA2IU5    JALAP/Y 


Fig.   130. — Blending  inheritance  ("incomplete  dominance")  in  Mirabilis  jalapa,  showing 
1:2:1  ratio  of  three  genotypes  in  F%.     (Adapted  from  Correns.) 

four  combinations  are  possible :  a  T  sperm  with  a  T  egg,  a  T  sperm  with  a 
t  egg,  a  t  sperm  with  a  T  egg,  and  a  t  sperm  with  a  t  egg.  These  four 
combinations  result  respectively  in  a  tall  plant  (pure  dominant,  TT),  two 
tall  hybrids  (Tt  and  tT),  and  a  dwarf  plant  (pure  recessive,  it).  It  is 
obvious  that  in  the  long  run  these  three  types  will  occur  in  the  ratio  of 
1:2:1. 

Mendel's  researches  on  peas  included  also  a  study  of  six  other  pairs 
of  heritable  characters  (now  known  as  allelomorphic  pairs),  the  two 
members  of  each  pair  behaving  toward  each  other  in  a  manner  similar  to 
that  described  above  for  tallness  and  dwarfness.  He  further  observed 
that  the  seven  pairs  are  entirely  independent  of  each  other  in  inheritance 
(Mendel 's  second  law;  now  modified;  see  p.  384).  All  these  phenomena  he 
interpreted  on  the  basis  of  the  hypothesis  that  each  character  is  in  some 
way  represented  by  a  factor  in  the  cells,  new  combinations  of  factors 


MENDELISM  AM)  M UTATIOA  339 

being  formed  at  fertilization  and  the  members  of  each  allelomorphic  pair 
of  factors  separating  when  the  genu  cells  arc  formed. 

The  Mendelian  proportion  of  pure  forms  and  hybrids  is  more  easily 
followed  in  eases  of  "incomplete  dominance*'  the  pure  dominants  here 
being  visibly  distinguishable  from  the  hybrids.  Such  a  case  is  thai  of 
Mirabilis  jalapa,  the  four-o'clock  (Fig.  130).  If  plants  bearing  pure  red 
flowers  (var.  rosea)  are  crossed  with  (hose  bearing  pure  while  flowers 
(var.  alba)  the  result  is  an  Fj  generation  of  intermediate  pink-flowered 
plants.  When  these  pink  hybrids  are  bred  among  themselves  the  result- 
ing F2  generation  comprises  plants  of  three  visibly  different  types:  pure 
dominants  with  red  flowers,  hybrids  with  pink  flowers,  and  pure  recessives 
with  white  flowers,  in  the  numerical  ratio  of  1:2:1. 

Terminology. — We  may  here  introduce  certain  terms  prominent  in  the 
literature  of  genetics.  The  genotype  is  the  entire  assemblage  of  factors  which  an 
organism  actually  possesses  in  its  constitution,  irrespective  of  how  many  of 
these  may  be  expressed  in  externally  visible  characters.  The  phenotype  is  the 
aggregate  of  externally  visible  characters,  irrespective  of  any  other  factor-, 
unexpressed  in  characters,  which  may  be  present  in  the  organism.  For  illustra- 
tion: in  the  case  of  the  tall  and  dwarf  peas  there  are  in  the  second  hybrid  genera- 
tion (F2)  three  genotypes  (with  respect  now  only  to  the  single  character  pair 
discussed):  TT,  Tt,  and  tt,  represented  respectively  by  pure  tall  plants,  tall 
hybrids,  and  dwarfs;  but  there  are  only  two  phenotypes:  tall  and  dwarf,  because 
of  the  fact  that  the  complete  dominance  of  tallness  over  dwarfness  renders  the 
hybrids  externally  indistinguishable  from  the  pure  tall  individuals.  Thus  one 
phenotype  (tall  plants)  here  includes  individuals  with  two  genotypic  constitu- 
tions, and  the  two  can  be  distinguished  only  by  a  study  of  their  progeny.  In 
Mirabilis,  however,  there  are  in  the  F>  generation  not  only  three  genotype- 
represented,  but  also  three  phenotypes,  since  the  incomplete  dominance  renders 
the  hybrids  externally  unlike  either  of  the  pure  forms. 

An  individual  is  said  to  be  homozygous  for  a  given  allelomorphic  character 
pair  if  it  has  received  the  same  factor  from  the  two  parents — a  pea,  for  example, 
with  the  constitution  TT  or  tt.  If  it  has  both  members  of  the  pair,  such  as  77. 
it  is  said  to  be  heterozygous.  It  may  be  homozygous  for  some  allelomorphic  pairs 
and  heterozygous  for  others,  or  it  may  conceivably  be  either  homozygous  or 
heterozygous  for  all  of  its  characters.  Thus  an  organism  with  the  genotypic 
constitution  AA Bbcc  is  homozygous  for  the  characters  represented  by  .1.1  and 
re,  and  heterozygous  for  those  represented  by  Bh.  It  is  thus  a  pure  dominant 
with  respect  to  A  and  a,  a  pure  recessive  with  respect  to  C  and  c,  and  a  hybrid 
with  respect  to  B  and  b.  The  phenotypic  appearance  of  the  organism  would  be 
determined  by  the  dominant  factors  A  and  B  and  by  the  recessive  c;  a  given 
dominant  factor  dominates  only  its  recessive  allelomorph,  and  not  the  recessive 
factors  belonging  to  other  pairs.  It  is  a  common  practice  to  represent  dominant 
factors  or  characters  by  capital  letters  and  their  respective  recessive  allelomorphs 
by  the  corresponding  small  letters. 

The  Cytological  Basis  of  Mendelism-  Having  before  us  some  of  the 
principal  facts  of  Mendelism  and  Mendel's  interpretation  of  them,  we 


340 


INTRODUCTION  TO  CYTOLOGY 


may  now  turn  to  the  cytological  basis  of  the  Menclelian  phenomena,  and 
inquire  what  visible  mechanism  there  is  in  the  cell  which  will  in  any  way 
help  us  toward  an  understanding  of  the  striking  behavior  of  the  Mendelian 

characters. 

The  behavior  of  the  chromosomes  at  the  critical  stages  of  the  life 
cycle  as  described  in  the  chapters  on  reduction  and  fertilization  must 
first  be  recalled.  (See  Fig.  131.)  It  has  been  shown  that  their  history 
is  as  follows.  Each  parent  furnishes  the  offspring  with  a  set  of  chromo- 
somes, the  two  sets  (represented  in  the  diagram  by  ABCD  and  abed) 
being  associated  in  all  the  cells  of  the  offspring.  When  gametes  (or 
spores  followed  later  by  gametes  in  the  case  of  higher  plants)  are  to  be 


FERTILIZATION 
Union  of  simplex  groups 


CLEAVAGE 
Duplex  group i 
ABCD  abed 


SOMATIC  DIVISIONS 
Duplex  groups 


Aa     Bb      Cc     Dd 
SYNAPSI S 


GERM  CELLS 
Simplex  groups 


Fig.   131. —  Diagram  showing  tho  history  of  the  chromosomes  in  the  typical  life  cycle  of 

animals.      (After  Wilson,  1913.)      See  also  Fig.  77. 

formed  by  the  new  individual  the  chromosomes  pair  two  by  two  (synap- 
sis), the  two  homologous  members  of  each  pair  coming  from  the  two 
parental  sets.  In  the  first  maturation  division  (usually)  the  two  members 
of  each  pair  separate  and  enter  different  daughter  cells:  this  is  reduction, 
or  the  separation  of  entire  chromosomes,  presumably  qualitatively  differ- 
ent, instead  of  qualitatively  similar  halves  of  chromosomes  as  in  somatic 
division.  In  the  second  maturation  division  all  the  chromosomes  split 
longitudinally  (equationally),  so  that  as  the  result  of  the  two  divisions 
there  are  four  gametes  (or  spores),  two  of  them  differing  from  the  other 
two  in  chromatin  content.  The  somatic  chromosomes  are  therefore 
segregated  into  two  unlike  groups :  each  gamete  (or  spore)  has  a  single  set 
of  chromosomes,  the  set  being  composed  of  one  member  of  each  of  the 
pairs  formed  at  synapsis.  This  set  represents  the  contribution  made  to 
the  following  generation. 


MENDELISM  AND  MUTATION 


341 


It  will  be  recognized  at  once  thai  the  above  is  precisely  the  sort  of 


distribution  shown  by  i  he  characters  in  Mendel'* 


s  experiments:  two  groups 


PARENTS 


/ 


«k 


KQ,^l- 


Fig.   132.— Mendelian  inheritance  in  black  and  albino  gui 


guinea  pigs. 


Fig.  133.— Chromosome   history  in   the   cross   represented   in    Fig.    L32    showing  th» 

Parallehsm  ,    twl,M1  t       distribution  of  a  single  homologous  pair  of  chr ome°       M 

of  a  single  allelomorphic  pair  of  Mendelian  characters. 

of  (factors  for)  characters  are  brought  together  al  fertilization  and  -ire 
associated  in  the  body  of  the  offspring.    When  the  germ  cells  are  formed 


342  INTRODUCTION  TO  CYTOLOGY 

the  (factors  for  the)  two  characters  forming  each  allelomorphic  pair 
separate  and  pass  to  different  gametes  (or  spores).  Thus  the  chromo- 
somes and  the  characters  alike  form  a  duplex  group  in  the  body  cells  and 
a  simplex  group  in  the  gametes  (or  spores) :  the  chromosomes,  like  the 
characters,  form  new  combinations  at  fertilization  and  are  segregated 
when  the  gametes  (or  spores)  are  formed.  In  the  diagram  the  letters 
ABCDabcd  stand  equally  well  either  for  chromosomes  or  for  characters. 
In  view  of  these  facts  it  appears  extremely  probable  that  chromosomes 
and  Mendelian  characters  have  a  definite  causal  relationship  of  some 
kind:  it  is  scarcely  conceivable  that  the  exact  and  striking  parallelism 
that  they  show  can  be  without  significance. 

The  precise  nature  of  this  correspondence  between  chromosome  be- 
havior and  character  distribution  can  be  even  more  clearly  shown  by  a 
consideration  of  the  history  of  a  single  homologous  pair  of  chromosomes 
in  a  typical  Mendelian  cross.  If  a  pure  white  (albino)  guinea  pig  be  mated 
to  an  individual  of  a  pure  black  strain  the  offspring  are  all  black;  black 
is  completely  dominant  over  white  (Fig.  132).  If  these  black  hybrids 
are  bred  among  themselves  they  produce  in  the  F2  generation  three  black 
animals  to  one  white,  or,  more  precisely,  one  pure  black  to  two  black 
hybrids  to  one  pure  white.  Let  us  now  follow  a  single  pair  of  chromo- 
somes of  each  of  the  original  animals  through  these  two  generations. 

At  the  left  in  Fig.  133  are  represented  the  two  animals,  pure   black 
and  pure  white,  their  chromosomes  being  drawn  in  solid  black  and  outline 
respectively.    In  the  black  animal  the  two  chromosomes  pair  at  synapsis 
and  separate  to  the  two  daughter  cells  at  the  first  maturation  mitosis, 
and  split  longitudinally  at  the  second,  so  that  each  of  the  gametes  re- 
ceives a  single  chromosome  representing  a  longitudinal  half  of  one  of  the 
original  pair.     A  similar  process  occurs  in  the  white  individual.     Unions 
between  the  gametes  of  the  two  animals  now  result  in  the  Fx  hybrids, 
each  of  which  has  one  chromosome  from  its  black  parent  and  one  from 
its  white  parent  (not  counting  the  chromosomes  of  other  pairs).     When 
these  hybrids  form  gametes,  as  is  seen  at  once  in  the  diagram,  the  pa- 
ternal and  maternal  members  of  the  chromosome  pair  separate,  with  the 
result  that  half  the  gametes  receive  one  of  them  and  half  the  other. 
There  are  thus  two  kinds  of  spermatozoa  and  two  kinds  of  eggs,  one  kind 
carrying  the  paternal  chromosome  and  the  other  carrying  the  maternal 
one.     Chance  combinations  now  result  in  a  generation  (F^)  of  animals, 
one-quarter  of  which  have  derived  both  chromosomes  of  the  pair  in 
question  from  the   black  grandparent,  one-half  of  which  have  derived 
one  chromosome  of  the  pair  from  each  grandparent,  and  one-quarter  of 
which  have  derived  them  both  from  the  white  grandparent.     Moreover, 
these  animals  are  respectively  pure  black,  hybrid  black,  and  pure  white, 
in  the  proportion  of  1:2:1.     Thus  it  is  seen  that  there  is  a  direct  'paral- 
lelism, not  only  between  chromosome  sets  and  character  groups,  but  also 


MEXDEIJSM  AND  MUTATION 


343 


between  the  distribution  of  a  given  homologous  pair  of  chromosomes  and 
thai  of  a  single  allelomorphic  pair  of  Mendelian  characters. 

This  is  exactly  the  condition   which   would   result    it    two    material 
units,  each  representing  one  of  the  characters  of  an  allelomorphic  pair, 

were  located  in  two  homologous  chromosomes  that  pair  and  separate  at 
reduction.  The  chromosomes  afford  precisely  the  type  of  mechanism 
required  to  account  for  the  distribution  of  characters  if  the  latter  arc 
associated  with  a  definite  material  basis.  It  is  this  parallelism  between 
the  behavior  of  the  chromosomes  in  reduction  and  that  of  Mendelian 
factors  in  segregation,  first  emphasized  by  Boveri  and  by  Sutton,  which 
has  led  geneticists  generally  to  the  view  that  the  characters  arc  actually 


Fig.   1.34. — Diagram  showing  the  16  genotypic  constitutions  which  may  be  present  in  the 
gametes  of  an  organism  with  only  4  pairs  of  factors.     (After  Wilson,  L913 

represented  in  the  chromosomes  by  material  factors,  or  genes,  which  in 
some  unknown  manner  control  the  development  of  the  characters  in 
the  body. 

The  earlier  view  that  each  character  is  thus  represented  by  a  single 
material  unit  or  determiner  has  now  given  way  to  the  more  fully  devel- 
oped Factorial  Hypothesis,  according  to  which,  on  the  one  hand,  a 
character  may  be  due  to  the  cooperative  action  of  two  or  more  factors 
("duplicate"  or  " cumulative "  factors);  and,  on  the  other  hand,  a  single 
factor  may  have  "manifold  effects,"  influencing  the  development  of  several 
characters.  The  factors,  or  genes,  are  thought  by  some  to  constitute  a 
complex  reaction  system,  interactions  between  genes  having  a  marked 
effect    upon    their    activity   in    producing   characters.1     "The   factorial 

1  A  simple  and  brief  explanation  of  the  effects  of  cumulative  factors  is  given  by 
Coulter  and  Coulter  (1918). 


344  INTRODUCTION  TO  CYTOLOGY 

hypothesis  does  not  assume  that  any  one  factor  produces  a  particular 
character  directly  and  by  itself,  but  only  that  a  character  in  one  organism 
may  differ  from  a  character  in  another  because  the  sets  of  factors  in  the 
two  organisms  have  one  difference."  "It  can  not  .  .  .  be  too  strongly 
insisted  upon  that  the  real  unit  in  heredity  is  the  factor,  while  the  charac- 
ter  is  the  product  of  a  number  of  genetic  factors  and  of  environmental 
conditions"  (Morgan  et  at,  1915,  pp.  210,  212). 

The  abundant  opportunity  for  the  formation  of  new  factor  combina- 
tions should  be  noted  in  this  connection.  An  organism  with  four  pairs  of 
chromosomes  in  its  body  cells,  and  only  one  pair  of  factors  in  each  chromo- 
some pair,  could  form,  as  the  result  of  the  independent  distribution  of  the 
four  pairs  of  chromosomes,  gametes  with  as  many  as  16  different  geno- 
typic  constitutions  (Fig.  134).  Such  a  diversity  being  present  in  the 
gametes  of  both  sexes,  this  means  that  more  than  200  different  com- 
binations are  possible  at  fertilization.  The  12  pairs  of  chromosomes  in 
man  may  in  the  same  way  form  several  million  such  combinations.  Since 
there  is  good  reason  to  believe  that  each  chromosome  carries  more  than 
one  factor  the  number  of  variations  actually  produced  by  these  means  is 
almost  incalculable.  This  subject  will  be  pursued  further  in  the  chapter 
on  Linkage  (Chapter  XVII),  where  the  evidence  for  the  presence  of  many 
factors  in  a  single  chromosome  will  be  presented  and  the  consequences  of 
this  condition  pointed  out. 

MUTATION 

Although  opinion  is  divided  over  the  question  of  the  real  nature  of  the 
phenomenon  of  mutation,  particularly  in  (Enothera  Lamarckiana,  one 
school  (deVries  et  at.)  holding  that  it  represents  the  actual  origin  of  new 
forms,  and  another  (Bateson,  Davis,  Lotsy)  regarding  it  as  the  result  of 
segregation  in  an  organism  of  hybrid  constitution,  the  observed  facts  in 
either  case  are  nevertheless  very  significant  with  respect  to  the  chromo- 
some theory  of  heredity.  The  mutations  observed  to  arise  from  (Enothera 
Lamarckiana  fall  into  two  general  classes:  first,  those  accompanied  by 
alterations  of  the  normal  chromosome  number  (seven  pairs),  and  second, 
those  in  which  the  number  undergoes  no  change.1 

Mutations  Accompanied  by  Changes  in  Chromosome  Number. — It 
is  to  be  noted  first  of  all  that  the  mutants  belonging  to  this  class  do  not 
behave  in  a  typically  Mendelian  fashion  when  bred  to  other  forms,  and 
that  this  is  correlated  with  the  serious  disturbance  of  the  chromosome 
mechanism.  (Enothera  mutants  with  many  abnormal  chromosome  num- 
bers have  been  observed;  Gates,  for  example,  found  them  with  15,  20, 
21,  22,  23,  27,  28,  29,  and  30  chromosomes. 

1  Our  knowledge  of  the  cytology  of  the  Oenotheras  is  due  mainly  to  the  researches 
of  Gates,  Davis,  Stomps,  and  Miss  Lutz. 


MENDEL/SU  AM)  MUTATION 


345 


The  2$-chromosome  Mutants  (gigas  group).  In  the  mutanl  tonus  of 
tin's  group,  of  which  (Enothera  gigas  is  :i  member,  the  somatic  number  of 
chromosomes  is  28  ra1  her  1  lian  I  1 ;  t  he  plants  are  te1  raploid  I  Fig.  135,  B). 
How  this  condition  arises  is  not  certainly  known.  Stomps  (1912 
believed  it  to  he  the  result  of  1  he  union  of  1  wo  unreduced  gametes,  whereas 
Gates  (19096)  suggested  thai  'the  doubling  in  the  chromosome  number 
had  probably  occurred  as  the  result  of  a  suspended  mitosis  in  i he  feii ilized 
egg  or  in  an  early  division  of  the  young  embryo."  Strasburger  (19106) 
also  adopted  the  latter  view. 


*»y  e 


Fk;.   L35. — Chromosomes  in  (Enothera  mutants. 

A,  interkinesis  in  (E.  Lamarck i ana;  7  split  chromosomes.  />',  same  in  (E.  gigas;  11 
split  chromosomes.  C,  somatic  cell  of  CE.  semilata;  1")  chromosomes.  I),  tnetaphase  of 
homceotypic  mitosis  in  CE.  biennis  lata,  showing  8  chromosomes  on  one  spindle  and  7  on 
the  other.  Spores  and  gametes  with  these  numbers  will  result.  E,  the  21  chromosomes 
in  a  mutant  from  (E.  Lamarckiana.  (A  and  Ii  after  Davis,  1911;  C  and  I)  after  Gates  ami 
Thomas,  1914;  E  after  Lute,  1912.) 


The  mutants  of  the  gigas  group  are  characterized  chiefly  by  an  unusu- 
ally large  size,  not  only  of  the  plant  as  a  whole  but  also  of  its  anatomical 
eoustit uents.  In  t he  tet raploid  mutant  (Enothera  stenona  res,  tot  instance, 
Tupper  and  Bartletl   (1916)  found  that  the  change  from  the  diploid  to 

the  tetraploid  condition  is  concomitant  with  a  50  per  cent  increase  in 
the  length  of  the  vessel,  a    150  per  cent    increase  in   t  he  area  of  its  cross 

section,  a  50  per  cent  increase  in  the  length  and  diameter  of  the  tracheids, 
an  increase  in  t  he  t  hree  dimensions  of  t  he  medullary  ray  cells,  and  a  break- 
ing up  of  the  tall  multiple  ray  into  a  number  of  thin  simple  cays. 


346  INTRODUCTION  TO  CYTOLOGY' 

A  further  significanl  observation  on  mutants  of  this  type  was  thai  of 
Gregory  (1914)  on  a  tetraploid  Primula.  He  showed  by  breeding  experi- 
ments thai  twice  the  normal  number  of  Mendelian  factors  are  present: 
thus  when  the  chromosome  number  is  tetraploid  the  number  of  factors  is 
also  tetraploid;  each  allelomorphic  pair  is  represented  twice. 

It  should  be  stated  that  not  all  cases  of  gigantism  are  accompanied 
in  this  manner  by  an  increase  in  the  chromosome  number.  In  Phragmites 
communis,  for  example,  Tischler  (1918)  finds  abnormally  large  size  asso- 
ciated with  an  increase  in  the  size  of  the  chromosomes,  but  not  in  their 
number.  Stomps  (1919)  points  out  that  among  gigas  mutants  of  (Eno- 
thera, Narcissus,  and  Primula  there  are  diploid  as  well  as  tetraploid 
individuals,  which  must  mean  that  the  altered  chromosome  number  is  not 
the  sole  cause  of  such  mutation  but  is  rather  one  of  the  characters  of  the 
mutant. 

The  lb-chromosome  Mutants  {lata  group). — The  presence  of  an  extra 
chromosome  in  the  cells  of  (Enothera  lata  and  other  members  of  this  group 
is  due  to  the  fact  that  the  members  of  one  pair  of  chromosomes  in  (Eno- 
thera Lamarckiana  fail  to  separate  at  the  reduction  division,  both  of  them 
going  to  one  daughter  cell.  This  phenomenon  is  known  as  non-disjunc- 
tion. As  a  consequence  there  are  gametes  with  eight  and  six  chromosomes 
rather  than  the  normal  seven;  and  a  union  of  an  8-chromosome  gamete 
with  a  normal  7-chromosome  gamete  results  in  an  individual  with  15 
chromosomes  instead  of  the  normal  14  (Fig.  135,  C). 

In  her  study  of  15-ehromosome  mutants  Miss  Lutz  (1917)  found  11  or 
12  types  belonging  to  this  group;  only  two  of  them  were  of  the  usual  lata 
type.  This  condition  may  be  accounted  for  on  the  hypothesis  that  it  is 
sometimes  one  pair  of  chromosomes  and  sometimes  another  which  fails 
to  separate  at  the  time  of  reduction,  so  that  the  extra  chromosome  is  not 
in  all  cases  the  corresponding  one  of  the  complement.  If  the  various 
chromosomes  of  the  complement  differ  in  hereditary  value,  as  there  is 
much  reason  to  believe,  it  is  evident  that  this  would  allow  for  a  great 
variety  of  mutants  with  the  same  aberrant  chromosome  number.  In 
(Enothera  scintillans  Hance  (1918)  has  shown  by  careful  measurements 
that  the  extra  chromosome  can  be  distinguished  from  the  regular  14. 
Two  classes  of  gametes  are  formed,  some  with  seven  chromosomes  and 
some  with  eight  (Fig.  135,  D)  The  union  of  two  7-chromosome  gametes 
gives  (Enothera  Lamarckiana,  the  form  from  which  (E.  scintillans  sprang 
as  a  mutant;  whereas  a  union  of  a  7-chromosome  gamete  with  an  8- 
chromosome  gamete  gives  (E.  scintillans.  Hance  therefore  points  out 
that  the  scintillans  characters  are  plainly  associated  with  the  extra 
chromosome.  <E.  scintillans  was  further  observed  to  give  rise  to  a  type 
resembling  (E.  oblonga.  It  is  possible  that  this  was  due  to  the  union  of 
two  8-chromosome  gametes. 


MENDELISM  AND  MUTATION  347 

The  21-chromosome  Mutants  (semigigas  group).  The  21-ehromosome 
condil  ion  is  brought  about  by  tin'  union  of  a  normal  7-chromosorae  gamete 
with  a  14-chromosome  gamete  produced  either  by  a  mutanl  of  the  gigas 
group  or  by  a  normal  plant  through  failure  of  reduction.  These  mutants 
are  triploid  (Fig.  135,  E). 

As  might  be  expected  in  forms  with  aberrant  chromosome  numbers, 
especially  in  those  with  an  odd  number  like  21,  certain  irregularities 
occur  in  the  maturation  mitoses,  with  the  result  that  gametes,  and  hence 
progeny,  with  abnormal  chromosome  numbers  are  produced.  Accom- 
panying this  irregular  behavior  is  an  unsettled  hereditary  condition,  the 
plants  showing  various  unusual  character  combinations  and  differing 
markedly  from  generation  to  generation.  In  the  course  of  such  a  series 
of  generations  there  is  a  gradual  settling  down  to  the  normal  number 
through  the  loss  of  chromosomes  in  irregular  mitoses.  When  the  nor- 
mal number  (14)  is  finally  reached  the  plants  become  much  more  stable 
in  their  hereditary  behavior:  the  hereditary  mechanism  again  appears 
to  be  in  equilibrium.  In  one  such  series  of  mutant  forms,  which  had 
arisen  in  the  first  place  from  CEnolhera Lamar cki ana,  deVries  and  Stomps 
found  that  after  the  normal  chromosome  number  had  thus  been  settled 
upon  the  plants  were  not  of  the  Lamarcki ana  type.  Although  the  num- 
ber was  that  characteristic  of  the  original  Lamarckiana  individual  from 
which  the  series  originated,  the  assortment  was  apparently  a  new  one: 
during  the  settling  down  process  some  chromosome  pairs  of  the  com- 
plement had  been  lost  completely  while  others  had  been  duplicate*  1. 
The  plants  consequently  had  certain  characters  represented  in  duplicate, 
while  others  present  in  the  orignal  ancestral  plant  were  entirely  lacking. 
Upon  the  theory  that  the  chromosomes  of  the  complement  differ  in  here- 
ditary effect,  the  above  facts  are  readily  explained. 

Conclusion.— All  the  evidence  goes  to  show  that  in  the  above  described 
mutations  the  change  in  chromosome  number  and  the  change  in  the  visible 
characters  of  the  organism  occur  simultaneously.  This  fact  const  itutes  a 
strong  support  to  the  theory  that  in  the  chromosomes  there  are  factors 
representing  heritable  characters,  and  indicates  that  mutations,  whatever 
may  be  their  ultimate  nature,  are  causally  connected  with  alteration-. 
often  visible,  in  the  cell  mechanism. 

Bearing  on  the  Origin  of  Species  and  Varieties. --The  question  nat- 
urally follows  as  to  what  extent  the  origin  of  new  species  and  varieties 
may  be  bound  up  with  fluctuations  in  chromosome  number.  Although 
the  experimental  evidence,  aside  from  that  of  the  (Enothera  mutants 
which  many  do  not  regard  as  species  at  all.  is  as  yet  quite  meager,  the 
following  facts  are  nevertheless  very  suggestive  in  this  connection. 

In  published  lists  of  chromosome  numbers  it  is  strikingly  evident 
that  the  numbers  shown  by  the  species  of  a  given  genus  or  even  of  an 
entire  familv  very  commonly  form  a  series  of  multiples.     Also,  one  or 


348 


INTRODUCTION  TO  CYTOLOGY 


two  species  of  such  a  group  frequently  have  but  one  or  two  chromosome 
pairs  more  or  less  than  one  of  the  numbers  of  the  multiple  series:  From 
1  his  it  is  to  be  inferred,  as  suggested  by  MeClung  (1905,  1907),  that  there 
is  a  relationship  of  some  sort  between  the  constitution  of  the  chromosome 
complement  and  the  externally  visible  taxonomic  characters.  Certain 
illustrative  examples  will  now  be  cited. 

Plants. — In  the  Leguminosae  most  of  the  species  which  have  been 
examined  possess  either  6,  12,  or  24  pairs;  Pisum  has  7.  Common 
numbers  in  the  Rosacea?  are  8,  16,  and  32;  some  species  have  6. 
In  a  new  study  of  a  large  number  of  species  of  Rosa  Tackholm  (1920) 
finds  the  fundamental  haploid  chromosome  number  in  this  genus  to 
be  seven  rather  than  eight.  The  various  species  of  Chrysanthemum 
have  9,  18,  27,  36,  and  45  pairs.  In  both  Triticum  (Sakamura  1918)  and 
Avena  (Kihara  1919)  the  pairs  number  7,  14,  and  28  (Fig.  136).     From 


a  b  c 

Fig.   136. — The  chromosomes  of  3  species  of  Avena.     a,  A.  strigosa;  14  chromosomes,     b, 
A.  barbata;  28  chromosomes,     c,  A.  sterilis;  42  chromosomes.      (After  Kihara,  1919.) 

these  and  many  other  similar  cases  it  is  inferred  that  tetraploid  species 
have  been  derived  from  diploid  species  in  much  the  same  way  that 
Oenothera  gigas  with  its  14  pairs  has  been  observed  to  arise  from  (E. 
Lamarckiana  with  seven;  that  triploid  species  have  arisen  either  b}r 
dispermy  or  by  a  union  of  diploid  and  haploid  gametes  as  in  the  semigigas 
group  of  (Enothera  mutants;  that  hexaploid  species  have  in  turn  arisen 
from  the  triploid  ones;  and  that  those  forms  with  numbers  not  belonging 
to  the  regular  multiple  series  have  resulted  from  further  irregularities  in 
chromosome  behavior.  Among  such  irregularities  are  the  failure  of  the 
two  members  of  a  homologous  pair  to  separate  at  reduction  (non-dis- 
junction), and  the  segmentation  of  certain  chromosomes  at  their  points 
of  constriction  (Sakamura  1920;  Kuwada  1919).  Changes  in  chromo- 
some number  are  thus  looked  upon  as  an  important  factor  in  the  origin 
of  new  species. 

The  above  conclusion  is  supported  further  by  the  observations  of 
Sakamura  (1918)  and  Kihara  (1919)  on  wheat.  Sakamura  finds  that  the 
one-grained  wheats  (Triticum  monococcum)  have  14  chromosomes  (dip- 
loid), the  emmer  wheats  (T.  dicoccum,  T.  polonicum,  T.  durum,  and  T. 
turgidum)  28  (tetraploid),  and  the  spelt  wheats  (T.  spelta,  T.  vulgar  e< 


MENDELISM  AND  MUTATION  349 

and  T.  compactum)  12  (hexaploid).  He  concludes  thai  the  one-grained 
wheats  are  the  ancestral  forms  from  which  the  emmer  and  spell  wheats 
have  arisen  through  changes  in  chromosome  number.  This  is  precisely 
the  conclusion  which  Schulz  (1913)  and  Zade  (1914,  1918)  had  reached 
on  other  grounds.  Kihara  (1919)  found  further  thai  by  crossing  emmer 
and  spelt  wheats  fertile  hybrids  with  35  chromosomes  could  be  obtained, 
and  that  these  in  future  generations  produced  forms  wit  h  varying  chromo- 
some numbers  because  of  the  irregular  manner  in  which  the  chromo- 
somes are  distributed  at  the  time  of  reduction.  (See  p.  253.)  &£  a 
general  rule  hybrids  produced  by  crossing  forms  with  different  chromo- 
some numbers  are  sterile,  but  when  they  are  fertile  and  their  chromosome 
number  is  odd  there  is  usually  an  irregularity  in  genetic  behavior  for 
several  generations  until  the  number  again  becomes  settled.  In  some 
cases  the  number  thus  settled  upon  is  that  of  the  original  ancestor  with 
the  lower  number  (the  (Enothera  mutants  of  deVries  and  Stomps  cited 
above),  whereas  in  other  cases  (Triticum)  it  is  that  of  the  ancestor  with 
the  higher  number.  The  manner  in  which  many  such  changes  in  number 
occur  is  not  yet  known. 

A  study  of  the  chromosomes  in  the  genus  Crepis  has  been  made  from 
this  point  of  view  by  Rosenberg  (1918,  1920).  He  finds  four  species, 
including  C.  virens,  with  three  pairs  of  chromosomes,  eight  species  with 
four,  four  species  with  five,  one  species  with  eight,  one  species  with  nine, 
and  three  species  with  21.  In  Crepis  the  chromosomes  differ  markedly 
in  size,  and  Rosenberg  concludes  that  the  species  with  three,  four,  and  five 
pairs  have  arisen  through  such  irregular  distribution  of  the  smaller  chro- 
mosomes as  has  actually  been  observed  in  the  maturation  division-. 
together  with  recombinations  occurring  at  fertilization.  The  segmenta- 
tion of  the  larger  chromosomes  of  the  complement  is  not  thought    to 

occur. 

In  an  extensive  investigation  of  the  chromosomes  of  Zea  Mays 
Kuwada  (1919)  has  found  cytological  confirmation  of  the  conclusion  of 
Collins  (1912)  that -this  species,  which  for  some  years  has  played  a 
conspicuous  role  in  genetical  investigations,  is  in  all  probability  a  hybrid 
between  Euchlcena  ?nexicana  (teosinte)  and  some  other  unknown  form 
belonging  to  the  nearly  related  Andropogonese.  Owing  to  their  in- 
equality in  size  Kuwada  is  able  to  distinguish  what  he  considers  to  be 
the  chromosomes  of  the  two  supposed  ancestral  derivations  in  the  cells 
of  certain  races  of  maize.  Thus  gemini  with  components  of  unequal 
size  are  frequently  observed  in  the  microsporocytes. 

Animals. — The  most  complete  descript  ion  of  the  chromosomes  in  alarge 
numberof  closely  related  animal  species  is  thai  given  by  Metzi  191 1.  19166) 
for  the  Drosophilidse.  In  about  30  species  Met  z  has  identified  no  less  than 
12  main  types  of  chromosome  groups,  all  but  one  of  them  being  found  in 
the  genus  Drosophila.     In  Fig.  137  are  shown  diagrannnat  ically  the    12 


350  INTRODUCTION  TO  CYTOLOGY 

principal  types  as  they  appear  with  their  characteristic  arrangements  at 
the  time  of  cell-division.  Type  A  is  that  found  in  Drosophila  melano- 
gaster  (formerly  known  as  D.  ampelophila) ,  upon  which  the  greater  part 
of  the  genetic  work  in  these  flies  has  been  done;  it  is  also  characteristic 
of  several  other  species. .  Here  there  are  two  pairs  of  large  bent  "euchro- 
mosomes,"  one  pair  of  sex  chromosomes,  and  one  pair  of  very  small 
"m-chromosomes."  In  some  of  the  other  species  it  is  seen  that  the 
position  of  one  or  both  of  the  large  pairs  is  occupied  by  two  pairs  but 


II         II*        II  o       || 


D 


$    I    6 


ni 


?   j  e 


(V 


6 

Fig.   137. — The   12  principal  types  of  chromosome  groups  found  in  the  cells  of  the  Droso- 

philidae.      {After  Metz,  1916.) 

half  as  large,  and  there  is  much  evidence  to  show  that  these  have  arisen 
by  a  segmentation  of  the  large  chromosomes.  Furthermore,  the  m- 
chromosomes  do  not  appear  in  some  species,  but  it  is  not  yet  certain 
whether  they  are  actually  lost  through  irregular  mitoses  or  are  fused 
with  some  of  the  larger  chromosomes  of  the  group.  Since  irregular 
mitoses  resulting  in  abnormal  distributions  of  chromosomes  are  actually 
observed  in  Drosophila,  and  are  known  to  be  accompanied  by  changes 
in  the  hereditary  constitution,  there  can  be  little  doubt  that  by  this  and 
other  means  all  the  typos  of  chromosome  groups  have  been  derived  from 


MENDELISM  AND  MUTATION  351 

one  or  two  original  types,  and  thai  the  specific  differences  exhibited  by 
the  organisms  arc  related  to  these  differences  in  their  chromosome 
complements. 

This  conclusion  is  supported  by  the  observations  of  McClung,  Robert- 
son, and  others  on  the  chromosomes  of  other  insect  families.  In  the 
grasshoppers,  for  example,  Robertson  (1916)  finds  that  the  various 
chromosomes  of  the  complement  form  a  regular  graded  series  and 
can  be  identified  in  all  the  species  and  genera  studied.  The  nearer  the 
relationship  the  more  nearly  similar  are  the  chromosome  complements. 
Robertson  states  that  the  degree  of  relationship  is  as  clearly  expressed 
in  the  nucleus  as  in  the  externally  visible  characters,  and  thai  the 
evidence  indicates  that  descent  by  variation  from  a  common  ancestral 
series  of  chromosomes  is  paralleled  by  degrees  of  variation  in  somatic 
structures. 

Mutations  Accompanied  by  No  Change  in  Chromosome  Number. 
In  most  of  the  examples  of  this  class  it  has  been  found  that  the  mutant 
behaves  in  a  strictly  Mendelian  manner,  usually  being  recessive  to  the 
type  from  which  it  sprang.  This  observation  falls  into  line  with  the  fad 
that  the  number  and  behavior  of  the  chromosomes  remain  the  same;  the 
operation  of  the  Mendelian  mechanism  is  not  disturbed.  Consequently 
if  the  origin  of  mutations  of  this  class  is  dependent  on  the  chromosomes 
it  must  be  due  to  a  change  of  some  kind  occurring  within  the  chromosome 
and  affecting  the  character  of  its  factors,  or  genes.  That  such  factor 
mutations  do  take  place  is  the  hypothesis  upon  which  a  large  school  of 
geneticists  is  attempting  to  account  for  many  of  the  observed  phe- 
nomena of  inheritance.  Such  mutations  may  involve  either  a  single 
gene  only  (" point  mutation")  or  a  group  of  genes  occupying  a  given 
region  of  a  chromosome  ("regional  mutation").  Furthermore,  they 
may  apparently  occur  either  in  the  germ  cells  or  in  the  somatic  cells. 
but  seem  to  be  most  frequent  in  the  former  at  the  time  of  maturation, 
for  the  reason  that  only  one  or  two  gametes  (or  spores  followed  later  by 
gametes  in  most  plants)  among  the  large  number  produced  reveal  the 
presence  of  the  altered  gene  in  their  effect  upon  the  offspring.  The 
mutation  in  the  gene  must  here  take  place  after  the  multiplication  of 
the  germ  cells  has  been  nearly  or  quite  completed;  otherwise  the  effed 
would  be  manifested  by  a  larger  number  of  gametes  (or  spores).  It.  ;i- 
is  true  of  the  majority  of  cases,  the  mutation  is  such  as  to  result  in  a 
recessive  character,  this  character  does  not  manifest  itself  until  it  meets 
a  similarly  mutated  gene  in  the  homozygous  individual.  Thus  such 
an  alteration  may  remain  latent  for  many  generations,  or  may  never 
come  to  expression  at  all. 

Factor  mutations  occurring  in  somatic  ( ineristematic)  cells  result  in 
what  are  known  as  "vegetative  mutations."  These  are  of  two  principal 
kinds:  bud  sports  and  chimeras.     In  the  case  of  the  bud  sport,  which  is 


352  INTRODUCTION  TO  CYTOLOGY 

believed  to  be  due  to  a  factor  mutation  in  the  very  young  bud,  the  entire 
product  of  the  bud— branch,  flower,  or  flower  cluster — has  a  new  geno- 
typic  constitution  and  exhibits  an  appearance  often  strikingly  different 
from  that  of  the  other  branches  or  flowers  of  the  plant.  Such  a  factor 
mutation  occurring  in  a  partially  developed  shoot  or  organ  results  in  a 
chimera,  in  which  a  distinct  portion  of  the  mature  structure,  commonly  a 
sharply  defined  sector,  differs  genotypically  and  in  appearance  from  the 
other  portions.  By  some  geneticists  vegetative  mutations  are  thought  to 
be  due  to  a  somatic  segregation  of  allelomorphic  factors  in  a  heterozygous 
individual  and  their  consequent  independent  activity  in  different  por- 
tions of  the  body. 

The  nature  of  the  change  which  may  thus  occur  in  the  gene  is  unknown, 
since  the  nature  of  the  gene  itself  is  entirely  a  matter  of  conjecture. 
Although  it  has  been  suggested  that  the  gene,  because  of  its  relative 
stability,  may  be  simply  a  molecule,  it  is  more  probably  a  more  complex 
colloidal  aggregate,  possibly  enzymatic  in  nature,  which  is  capable  of 
growth  and  division.  As  such  it  could  not  be  expected  to  be  absolutely 
stable,  as  some  geneticists  have  thought,  but  changes  would  in  all  likeli- 
hood take  place  occasionally  by  addition,  loss,  or  rearrangement  of  the 
constituent  atoms  of  the  molecule.  The  probable  rate  of  change  of  the 
genes  in  Drosophila  has  been  calculated  by  Muller  and  Altenburg  (1919). 
What  the  agencies  are  which  cause  such  changes  is  also  unknown.  Some 
evidence  has  been  brought  forward  to  show  that  genes  may  be  modified 
by  external  influences,  but  by  many  it  is  regarded  as  of  very  doubtful 
value.  The  stimuli  to  which  the  genes  respond  by  undergoing  some 
constitutional  change  are  probably  for  the  most  part  internal  ones. 
Although  the  number  of  ways  in  which  a  gene  may  change  is  limited  by 
its  own  organization,  the  possible  changes  are  nevertheless  numerous, 
so  that  it  is  very  probable  that  many  variations  which  constitute  initial 
steps  in  evolution  originate  in  this  manner.1 

Conclusion. — The  following  paragraphs  are  quoted  from  East  and 
Jones  (1919,  pp.  76  ff.): 

'The  relation  between  fact  and  theory  in  the  Mendelian  conception  of 
inheritance  is  this:  Various  kinds  of  animals  and  of  plants  were  crossed  and  the 
results  recorded.  With  the  repetition  of  experiments  under  comparatively 
constant  environments  these  results  recurred  with  sufficient  regularity  to  justify 
the  use  of  a  notation  in  which  theoretical  factors  or  genes  located  in  the  germ  cells 
replaced  the  actual  somatic  characters  found  by  experiment.  Later,  the  ob- 
served behavior  of  the  chromosomes  justified  localizing  these  factors  as  more  or 
less  definite  physical  entities  residing  in  them.  Now  the  data  from  the  breeding 
pen  or  the  pedigree  culture  plot  and  the  observations  on  the  behavior  of  the 
chromosomes  during  gametogenesis  and  fertilization  are  facts.  The  factors  are 
part  of  conceptual  notation  invented  for  simplifying  the  description  of  the  breed- 

1  See  the  discussion  of  these  points  by  Conklin  (1919-1920). 


MENDELISM  AND  MUTATION  353 

injr  facts  in  order  to  utilize  them  for  the  purposes  of  prediction,  just  as  the  chemi- 
cal atom  is  a  conception  invented  for  the  purpose  of  simplifying  and  making 
useful  observed  chemical  phenomena.  As  used  mat  hematically,  both  I  he  geneti- 
cal  factor  and  the  chemical  atom  are  concepts,  but  biological  data  lead  us  to 
believe  that  the  term  factor  represents  a  biological  reality  of  whose  nature  we 
are  ignorant,  just  as  a  molecular  formula  represents  a  physical  realit  \  of  a  nature 
yet  but  partly  known. 

"  With  this  distinction  in  mind,  one  may  treat  the  factor — or  the  atom — from 
two  points  of  view,  either  as  a  mathematical  concept  or  a  physical  reality.  As 
a  mathematical  concept  it  is  the  unit  of  heredity,  and  a  unit  in  any  notation  must 
be  stable.  If  one  describes  a  hypothetical  unit  by  which  to  describe  phenomena 
and  this  unit  varies,  there  is  really  no  basis  for  description.  He  is  forced  to 
hypothecate  a  second  fixed  unit  to  aid  in  describing  the  first. 

"  The  point  at  issue  in  this  connection  may  be  explained  as  follows:  Characters 
do  vary  from  generation  to  generation,  and  the  question  to  be  decided  is,  how 
much  of  this  variation  is  due  to  the  recombination  of  factors  (considered  now  as 
physical  entities)  and  how  much  is  due  to  change  in  the  constitution  of  the 
factors  themselves   ... 

"...  We  believe  there  should  be  no  hesitation  in  identifying  the  hypotheti- 
cal factor  unit  with  the  physical  unit  factor  of  the  germ  cells.  Occasional  changes 
in  the  constitution  of  these  factors,  changes  which  may  have  great  or  small 
effects  on  the  characters  of  the  organism,  do  occur;  but  their  frequency  is  not 
such  as  to  make  necessary  any  change  in  our  theory  of  the  factor  as  a  permanent 
entity.  In  this  conception  biology  is  on  a  par  with  chemistry,  for  the  practical 
usefulness  of  the  conception  of  stability  in  the  atom  is  not  affected  by  the  knowl- 
edge that  the  atoms  of  at  least  one  element,  radium,  are  breaking  down  rapidly 
enough  to  make  measurement  of  the  process  possible." 

Bibliography  at  end  of  Chapter  XVIII. 


23 


CHAPTER  XVI 

SEX 

In  the  present  chapter  attention  will  be  devoted  to  the  cytological 
aspects  of  the  inheritance  and  determination  of  sex.1  Much  that  is  not 
cytological  in  nature  will  enter  into  consideration,  but  it  is  far  from 
irrelevant:  it  has  a  direct  bearing  on  the  main  cytological  problem  and 
must  be  included  in  order  that  the  latter  may  be  placed  in  its  proper 
setting,  and  that  the  larger  problem  of  sex  may  not  be  misrepresented  by 
being  considered  only  from  the  cytological  point  of  view. 

From  early  times  few  essentially  biological  matters  have  been  of  more 
interest  to  man  than  that  of  the  determination  of  sex.  Until  recent 
years  this  interest  has  been  prompted  largely  by  practical  motives:  the 
ability  to  control  sex  in  man  and  his  domesticated  animals  is  something 
which  has  long  been  desired.  Of  the  many  early  ideas  entertained  on  the 
subject  the  majority  were  the  outcome  of  defective  generalization  and 
superstitious  conjecture,  and  may  be  encountered  in  thinly  veiled  form  at 
the  present  day,  but  a  review  of  them  all  would  be  out  of  place  here.  The 
modern  scientific  interest  in  the  problem  of  sex  is  far  from  being  a  purely 
practical  one.  The  great  bulk  of  recent  research  has  been  done  not 
merely  for  the  sake  of  the  practical  benefits  which  knowledge  in  this 
field  might  confer,  but  mainly  in  the  hope  that  it  may  lead  to  an  under- 
standing of  the  origin,  nature,  and  biological  significance  of  sex  itself, 
and  to  a  solution  of  some  of  the  problems  of  heredity.  For  this  reason 
studies  have  not  been  confined  to  man  and  his  economically  important 
animals;  any  animal  or  plant,  no  matter  how  obscure,  that  will  yield 
evidence  is  exhaustively  investigated,  and  there  can  be  no  doubt  that 
knowledge  gained  from  such  studies  will,  if  sound,  be  directly  applicable 
to  practical  ends. 

Experimental  Evidence  for  Sex-determination.— During  the  closing- 
decades  of  the  nineteenth  century  many  researches  were  carried  out  in  the 
hope  of  identifying  the  controlling  agency  in  sex-determination  with  one 
or  more  of  the  environmental  factors.  The  effects  of  light,  temperature, 
moisture,  and  nutrition  were  examined,  and  although  a  number  of  workers 
believed  their  methods  to  be  in  a  certain  measure  successful,  the  results 
were  on  the  whole  inconclusive.  Among  all  the  ideas  put  forward  the 
most  suggestive,  in  view  of  what  has  more  recently  been  ascertained, 

1  See  Correns  (1907),  Correns  and  Goldschmidt  (1913),  Morgan  (1913),  Doncaster 
(1914),  and  the  works  cited  at  the  beginning  of  the  preceding  chapter. 

354 


SEX  355 

was  that  of  Geddes  and  Thomson  (1889),  namely,  thai  the  two  sexes 
differ  primarily  in  the  character  of  their  metabolism,  the  female  sex 
being  characterized  by  the  preponderance  of  anabolic  processes  and  the 
male  sex  by  those  essentially  katabolic  in  nature.  This  conception  is 
important  not  only  in  connection  with  the  question  of  sex  control,  1  nit 
chiefly  with  respect  to  the  more  fundamental  problem  of  the  nature  of 
sex  itself. 

The  long  early  period  during  which  sex  was  looked  upon  as  a  character 
more  or  less  under  the  control  of  the  environmental  factor-  \\  as  succeeded 
by  one  in  which  it  came  to  be  regarded  as  something  automatically 
regulated  by  some  mechanism  or  condition  within  the  cell,  and  as  rela- 
tively unalterable  by  external  agencies.  This  conclusion,  definitely 
reached  by  Cuenot  (1899)  for  animals  and  by  Strasburger  (1900)  for 
plants,  received  the  support  of  a  number  of  experimental  researches  on 
animals  and  dioecious  plants.     Some  of  the  latter  will  first  be  mentioned. 

It  was  found  by  Blakeslee  (1906)  that  in  certain  strains  of  a  mold, 
Phycomyces,  there  are  produced  in  the  germ  sporangium  two  kinds  of 
asexual  spores,  which  give  rise  to  "plus"  and  "minus'  mycelia  respec- 
tively. The  "plus'1  (male?)  mycelium  later  produces  spores  which 
develop  only  into  "plus,:  mycelia  and  so  on  indefinitely,  while  the 
"minus"  (female?)  strain  perpetuates  only  the  "minus"  condition:  in 
both  cases  the  sex  seems  to  be  fixed  by  some  mechanism  functioning  at 
the  time  of  spore  formation. 

In  certain  dioecious  mosses  (fil.  and  Cm.  Marchal  1906,  1907)  two 
kinds  of  spores  are  produced  in  equal  numbers  in  the  capsule.  Those  of 
one  kind  develop  into  male  gametophytes  (bearing  antheridia  only)  and 
those  of  the  other  kind  produce  female  gametophytes  (with  archegonia 
only).  In  no  way  were  the  Marchals  able  to  alter  the  sexes  of  tin  s< 
plants.  Furthermore,  new  gametophytes  formed  by  regeneration  from 
the  old  ones  were  just  as  rigidly  fixed  as  to  sex.  Protonemata  regenerated 
from  the  tissue  of  the  sporophyte,  however,  gave  rise  to  leafy  branches 
bearing  both  antheridia  and  archegonia.  Both  sex  potentialities  were 
therefore  present  in  the  sporophytic  tissue  and  in  the  diploid  game- 
tophytes regenerated  from  it,  whereas  the  normal  haploid  gametophytes 
produced  from  spores  were  either  purely  male  or  purely  female.1  The 
gametophytes  of  Marchantia  (Noll;  Blakeslee  1906)  are  similarly  fixed 
as  to  sex:  if  propogated  repeatedly  from  gemmae  the  sex  in  any  given  line 
remains  the  same  in  spite4  of  alterations  in  the  environmental  condition-. 
In  Sphcerocarpos  Douin  (1909)  and  Strasburger  I  1909)  were  able  to  show 
that  two  spores  of  a  single  tetrad  produce  male  gametophytes  while  the 
other  two  produce  females. 

The  obvious  conclusion  to  be  drawn  from  the  above  cases  Is  that  in 
such  forms  a  separation  of  the  sexes  takes  place  during  sporogenesis. 

1  Diagrams  of  these  experiments  are  given  by  Morgan  (1919a,  pp.  152 


356  INTRODUCTION  TO  CYTOLOGY 

Both  sexes  are  represented  in  the  sporophyte  (diploid)  generation,  as 
shown  particularly  by  the  mosses,  but  the  spores,  though  morphologically 
similar,  are  of  two  distinct  kinds:  male-producing  and  female-producing. 
Since  it  is  precisely  at  sporogenesis  that  reduction  occurs,  the  natural 
inference  is  that  a  separation  of  qualitatively  different  sex-factors  of  some 
kind  occurs  in  the  heterotypic  division,  the  sexes  of  the  future  gameto- 
phytes  thus  being  automatically  determined. 

That  a  somewhat  similar  qualitative  difference  may  exist  in  the 
microspores  of  many  angiosperms,  resulting  in  the  frequent  dioecious 
condition  of  the  sporophyte,  was  concluded  by  Correns  (1907)  from  his 
researches  on  Bryonia  hybrids.  He  was  best  able  to  interpret  the 
phenomena  observed  on  the  hypothesis  that  the  eggs  are  all  similar  in 
having  the  female  sex  "tendency;"  that  there  are  two  kinds  of  micro- 
spores and  hence  two  kinds  of  male  gametes,  with  male  and  female 
"tendencies"  respectively;  and  that  in  the  sporophyte  the  male  tendency 
dominates  the  female.  Darling  (1909)  was  inclined  toward  a  similar 
conclusion  for  Acer  N  eg  undo. 

Strasburger  (1910)  in  a  general  discussion  of  the  subject  growing  out 
of  his  researches  on  Elodea,  Mercurialis,  and  other  plants,  summarized 
the  situation  in  plants  as  follows.  In  monoecious  mosses  the  separation 
of  the  sexes  occurs  in  the  somatic  divisions  at  the  time  the  sex  organs 
are  formed.  The  separation  has  been  secondarily  joined  with  reduction, 
so  that  in  the  derived  dioecious  mosses  it  occurs  at  sporogenesis.  In  the 
homosporous  pteridophytes  it  takes  place  at  some  stage  in  the  game- 
tophyte  before  the  formation  of  the  sex  cells,  as  in  monoecious  mosses, 
though  in  some  cases  (Equisetum,  Onoclea,  and  others)  a  marked  physio- 
logical dioecism  is  present.  In  heterosporous  pteridophytes  and  all 
seed  plants  the  gametophytes  are  dioecious  but  the  sporophytes  may  be 
either  monoecious  (hermaphroditic)  or  dioecious.  In  monoecious  forms  the 
sexes  are  separated  at  some  stage  prior  to  the  development  of  megaspores 
and  microspores,  whereas  in  dioecious  forms  it  must  take  place  at  some 
other  point  in  the  life  cycle,  since  the  two  kinds  of  sporophytes  (mega- 
spore-bearing  and  microspore-bearing)  are  distinct  from  their  initial 
stages.  There  is  some  evidence  to  show  that  such  dioecism  is  due  to  a 
differentiation  among  the  pollen  grains  and  hence  among  the  male 
gametes:  some  grains  have  a  strong  male  tendency  which  dominates  the 
female  tendency  of  the  egg,  male  progeny  resulting;  while  other  grains 
have  a  weak  male  tendency  dominated  by  the  female  tendency  of  the  egg, 
female  offspring  being  produced.  In  brief,  as  sex  separation  became 
joined  with  reduction  in  forms  with  monoecious  gametophytes,  the 
dioecism  of  the  gametophyte  and  heterospory  (first  physiological  and  then 
morphological)  followed;  and  this  in  turn  led  to  the  dioecism  of  the 
sporophyte  also.  Finally,  in  such  advanced  forms  there  appears  to  be  a 
differentiation  among  the  spores  of  one  sex,  the  microspores,  giving  male 
gametes  of  two  types.     The  sex   of   the   resulting  offspring  therefore 


SEX  357 

depends  here  upon  the  type  of  nude  gamete  functioning,  as  is  known 
to  be  the  case  in  so  many  animals.  It  lias  been  suggested  by  Allen  I  L919) 
that  the  separation  of  the  sex-factors  in  dioecious  seed  plants  may  possibly 
occur  in  the  division  which  differentiates  the  two  male  nuclei  in  the 
pollen  tube,  rather  than  at  the  divisions  producing  the  microspores. 
That  this  interpretation  cannot  be  applied  to  Mendelian  factors  in  general 
is  evidenced  by  the  fact  that  in  maize  hybrids  the  embryo,  with  very 
rare  exceptions,  has  been  found  to  be  like  the  endosperm  with  respect 
to  factors  introduced  by  the  pollen  parent.  So  far  as  these  factors  are 
concerned,  therefore,  the  two  male  nuclei  must  be  qualitatively  similar. 

Strasburger's  conclusion  regarding  monoecious  mosses  is  confirmed 
by  the  recent  experiments  of  Collins  (1919)  on  Funaria  hygrometrica.  In 
this  species  the  gametophytes  arising  from  spores  are  bisexual  (monoe- 
cious), but  if  gametophytes  are  produced  by  regeneration  from  the 
antheridia  or  perigonial  leaves  of  a  single  "male  flower,"  they  all  bear 
antheridia  only.  Collins  thinks  it  possible  that  dicecism  may  have 
arisen  as  a  result  of  vegetative  multiplication  following  such  a  somatic 
segregation  in  the  tissue  of  the  monoecious  gametophyte. 

In  animals  also  there  is  much  evidence,  aside  from  that  afforded  by 
the  chromosomes  to  be  discussed  below,  in  favor  of  the  view  that  sex  is 
internally  controlled.  The  following  illustrative  cases  may  be  cited. 
The  egg  of  the  bee  may  develop  either  parthenogenetically  or  after 
fertilization  by  a  spermatozoon:  in  the  former  case  a  male  (drone  |  results 
and  in  the  latter  a  female  (queen  or  worker,  depending  on  the  nature1  of 
the  food).  In  Phylloxera  (Morgan  1906,  1908,  1909,  1910)  there  are  two 
sizes  of  eggs  produced  by  the  females  of  the  second  parthenogenetic 
generation:  both  may  develop  parthenogenetically  after  forming  one 
polar  body,  the  larger  ones  into  females  and  the  smaller  into  males. 
Fertilized  eggs  always  develop  into  females.  In  Hydatina  (Whitney 
1914,  1916,  1917)  the  female-producing  eggs  form  one  polar  body  while 
the  male-producing  eggs  form  two.  Two  kinds  of  eggs  are  also  produced 
in  Dinophilus  (Malsen  1906;  Nachtsheim  1919).  but  in  this  form  both 
are  regularly  fertilized.  In  the  nine-banded  armadillo  (Newman  and 
Patterson  1909,  1910)  one  fertilized  egg  commonly  gives  rise  to  four  new 
individuals,  and  the  four  are  invariably  all  male  or  all  female.  Analogous 
instances  of  polyembryony  are  also  known  in  insects.  Human  twin-. 
if  "identical"  (produced  by  the  same  egg),  are  invariably  of  the  same 
sex;  if  "fraternal"  (produced  by  different  eggs)  they  may  or  may  not  be 
of  the  same  sex.  It  would  therefore  seem  that  sex  in  such  cases  &8  these 
must  be  determined  either  in  the  egg  before  fertilization  or  at  the  moment 

fertilization  occurs.1 

1  The  determination  of  sex  in  the  egg  before  fertilization,  as  m  Phylloxera  and 
Dinophilus,  is  termed  by  Haecker  '^progamic"  Bex-differentiation;  if  determination 
occurs  :it  the  moment  of  fertilization,  as  in  the  bee,  it  is  "syngamic"  Bex-differentia- 
tion; and  if  it  occurs  after  fertilization,  as  may  possibly  he  the  case  m  some  forms,  it 
is  "epigamic"  Bex-differentiation. 


358  INTRODUCTION  TO  CYTOLOGY 

Sex-Chromosomes.- -The  theory  of  the  automatic  determination  of 
sex  and  its  relative,  if  not  absolute,  fixity  has  had  one  of  its  strongest 
supports  in  the  results  of  certain  researches  on  the  spermatogenesis  of 
animals.  In  1891  Henking  noticed  in  certain  insects  that  half  of  the 
spermatozoa  contain  an  extra  body,  which  he  thought  might  be  a 
nucleolus.  It  was  subsequently  shown  by  Paulmier  (1899),  Montgomery 
(1901),  and  de  Sinety  (1901)  that  this  body  is  not  a  nucleolus  but  an 
extra  or  " accessory'  chromosome.  Henking's  misinterpretation  had 
apparently  been  due  to  the  fact  that  the  accessory  chromosome  often 
does  not  transform  into  a  portion  of  the  reticulum  along  with  the 
other  chromosomes  (" autosomes"),  but  remains  condensed  and  closely 
resembles  a  nucleolus.  Half  of  the  spermatozoa  in  these  animals  there- 
fore have  one  more  chromosome  than  the  others:  hence  the  male  is 
said  to  be  "heterogametic,"  or  "digametic."  It  was  at  once  suggested 
by  McClung  (1902)  that  the  accessory  chromosome  in  some  way 
determines  sex — that  eggs  fertilized  by  one  kind  of  spermatozoon 
develop  into  females,  while  those  fertilized  by  the  other  kind  become 
males.  This  represents  the  first  attempt  to  connect  a  given  character 
with  a  particular  chromosome.  An  extensive  series  of  researches  was 
now  undertaken  by  Wilson,  Miss  Stevens,  McClung,  and  a  number  of 
other  cytologists,  who  discovered  among  insects  many  striking  instances 
of  the  phenomenon.  Accessory  chromosomes  (also  referred  to  as  sex- 
chromosomes,  heterochromosomes,  idiochromosomes,  x-chromosomes, 
x-elements,  and  supernumerary  chromosomes)  of  a  number  of  different 
types  were  found,  not  only  among  insects,  where  they  are  best  displayed, 
but  also  in  certain  echinoderms,  nematodes,  mollusks,  and  vertebrates, 
including  birds  and  man.  A  number  of  representative  cases  will  now 
be  described. 

Male  Heterogametic. — In  the  threadworm,  Ascaris  (Boveri)  (Fig.  138) ! 
there  is  in  each  body  cell  and  primary  spermatocyte  of  the  male  a  single 
heterochromosome,  which  seems  to  be  attached  to,  or  to  constitute  a 
portion  of,  one  of  the  four  autosomes.  At  the  time  of  reduction  this 
passes  undivided  to  one  daughter  cell  at  the  first  division  and  divides  at 
the  second,  so  that  half  of  the  sperms  only  receive  it.  In  the  female  there 
are  two  such  heterochromosomes,  every  egg  receiving  one.  If,  now,  an 
egg  is  fertilized  by  a  sperm  without  a  heterochromosome  the  resulting 
individual  has  only  one  (that  from  the  egg)  and  develops  into  a  male. 

1  For  the  sake  of  brevity  and  clearness  these  diagrams  are  drawn  as  if  only  one 
maturation  mitosis  occurred  in  spermatogenesis  and  oogenesis.  It  will  be  understood 
that  there  are  two  divisions,  resulting  in  four  sperms  instead  of  the  two  shown,  and 
in  an  egg  and  three  polar  bodies  instead  of  the  two  eggs  shown.  The  diagrams  merely 
indicate  that  two  sorts  of  sperms  and  one  kind  of  egg  are  produced,  and  how  this 
is  brought  about.  In  the  cases  of  Lygceus  and  Prionidus  the  number  of  autosomes 
shown  (4)  is  not  the  actual  number  present.  See  the  review  of  the  subject  of  sex 
chromosomes  by  Wilson  (1911). 


SEX 


359 


If,  on  the  oilier  hand,  an  egg  is  ferl ilized  by  a  sperm  wit  b  a  heterochromo- 
sonie,  the  resulting  individual  receives  tw<>.  one  from  each  gamete,  and 
this  individual  develops  into  a  female. 

In  Ancyracanthus  (Mulsow  1912)  (Fig.  L38)  the  male  has  a  single 
heterochromosome  which,  since  it  has  do  homologue  with  winch  to  pair, 
passes  to  half  the  sperms,  while  in  the  female  there  arc  two  such  elements, 
every  egg  receiving  one.  The  two  types  of  union  result  in  individuals 
of  the  two  sexes,  as  in  Ascaris.  In  Ancyracanthus  Mulsow  stale-  thai 
the  five  and  six  chromosomes  can  actually  be  counted  in  the  living 
spermatozoa. 


ASCARIS 


ANCYRACANTHUS 


LYGAEOS 


PRlONIDUS 


Fig.   138. — The   behavior   of   the   sex-chromosomes   in    Ascaria    (Boveri),    AncyraeatUhua 
(Mulsow,  1912),  Lygaws  (Wilson,  1905),  and  Prionidua  (Payne,  1909). 

In  Lygcens  (Wilson  1905)  (Figs.  138;  139,  .4)  there  are  in  the  male 
two  heterochromosomes,  one  small  and  one  large  (an  UXY  '  pair);  in  the 
female  there  are  two  large  ones  ("XX").  Half  the  sperms  receive  the 
X  and  half  the  F,  and  every  egg  has  an  X.  Fertilization  by  an  X  sperm 
results  in  a  female  (XX),  and  by  a  Y  sperm  in  a  male  (A')'). 

In  Prionidus  (Payne  19091)  (Figs.  L38;  L39,  B)  the  male  has  three 
small  heterochromosomes  and  also  a  much  Larger  one.  At  the  time  of 
reduction  the  three  small  ones  behave  as  a  unit  and  pair  with  the  large 
one:  half  of  the  sperms  therefore  carry  the  former  and  half  the  latter. 
In  the  female  there  are  six  small  heterochromosomes,  and  the  eggs  are  all 
alike  in  having  three  each.  Fertilization  now  results  in  females  with 
six  small  elements  and  males  with  three  small  and  one  large. 

1  In  this  paper  Payne  gives  diagrams  of  several  other  types  of  heterochromosomes. 


360 


INTRODUCTION  TO  CYTOLOGY 


In  the  fruit  fly,  Drosophila  melanogaster  (Stevens  1907;  Morgan  1911; 
Metz  1914)  (Fig.  140),  there  are  four  pairs  of  chromosomes,  including  in 
the  male  an  17  pair  and  in  the  female  an  XX  pair.     Reduction  in 


#* 


B 


f\\ 


\ 


D 


' .  '•/ 


M h, 


E 


K 


•♦.♦ 


k 


F 


:  W 


Fig.   139. — Sex-chromosomes  in  various  insects. 

A,  spermatocyte  of  Lygceus,  showing  the  X-chromosome  at  left  and  y-chromosome 
above,  both  split.  X  2250.  (After  Wilson.)  B,  prophase  in  spermatocyte  of  Prionidus, 
showing  sex-chromosomes  enclosed  in  plasmosome.  X  2294.  (After  Payne.)  C,  pro- 
phase in  spermatocyte  of  Protenor.  X  2250.  (After  Wilson.)  D,  metaphase  of  hetero- 
typic mitosis  in  spermatocyte  of  Protenor.  X  2250.  (After  Wilson.)  E,  anaphase  of 
heterotypic  mitosis  in  spermatocyte  of  Musca  domestical  h,  heterochromosomes.  X  1500. 
(After  Stevens.)  F,  the  two  daughter  chromosome  groups  in  the  anaphase  of  the  hetero- 
typic mitosis  in  the  oocyte  of  Phragniatobia  fuliginosa,  showing  28-29  distribution.  X  4080. 
(After  Seiler.) 


spermatogenesis  gives  sperms  of  two  sorts :  all  contain  four  chromosomes, 
but  in  half  of  them  one  of  the  four  is  the  X,  and  in  the  other  half  it  is 
the  Y.  Since  every  egg  contains  an  X,  two  kinds  of  union  are  possible 
at  fertilization:  an  X  with  a  Y,  giving  a  male  fly,  and  an  X  with  an  X, 


SEX 


361 


giving  a  female.  In  this  case,  a  typical  example  of  the  A')'  form  of  sex 
inheritance,  extensive  researches  have  shown  thai  the  F-chromosome 
carries  no  factors  for  sex;  the  presence  of  one  ^-chromosome  is  associated 

with  maleness  and  t  hat  of  t  wo  A'-chroinosonies  with  femaleness.  Morgan 
(1914,  19196)  and  Morgan  and  Bridges  ( 1919)  find  that  gynandromorph- 
ism  frequently  appears  in  Drosophila  females  as  the  resull  of  the 
elimination  of  one  sex-chromosome  in  abnormal  mitosis. 


DBOSOPHILA 


/    \  /    \ 


MAN 


PhAA&MATOBIA 


Igpr 


Tig.   140. — The  behavior  of  the  sex-chromosomes  in  Drosoph ila  (Stevens,  Morgan,  Metz), 
Man  (Wieman),  Phragmatobia  (Seiler),  ami  the  fowl  (Guyer). 


Malone  (1918)  reports  that  there  are  present  in  the  spermatocyte  of 
the  dog  10  pairs  of  autosomes  and  one  large  unpaired  X-chromosome. 
The  X  passes  undivided  to  one  pole  in  the  first  mitosis  ami  divides  in  the 
second,  so  that  half  of  the  spermatids,  and  hence  spermatozoa,  receive 
an  X  while  half  do  not.  When  measurements  of  these  spermatozoa  are 
plotted  a  bimodal  curve  results,  showing  that  the  chromosome  difference 
is  correlated  with  a  size  dimorphism.  The  same  condition,  except  for 
the  number  of  autosome  pairs,  is  reported  for  the  spermatozoa  of  horses, 
pigs,  and  cattle  (Wodsedalek  1913,  1914,  1920). 

In  the  case  of  man  also  the  evidence  at  hand  indicates  a  digametic 
condition  on  the  part  of  the  male,  hut  certain  striking  discrepancies  in 
the  findings  of  various  investigators  have  afforded  a  puzzle  which  up  to 
the  present  time  has  not  been  satisfactorily  solved.  Plemming  (1898) 
counted  24  chromosomes  in  the  cells  of  the  cornea,  and  Duesberg  (  L906 
found  12  in  the  spermatocytes.  The  same  numbers  were  found  by 
Montgomery  (1912).  In  1910  Guyer  reported  that  the  spermatogonia 
of  the  negro  contain  22  chromosomes;  these  in  the  spermatocyte  form  10 
bivalents  and  two  distinguishable  accessories.  At  the  tirst  maturation 
mitosis  both  of  the  latter  go  to  one  pole  and  at  the  second  miiosi<  both 
divide,  so  that  half  of  the  sperms  have  10  chromosomes  and  half  have  L2; 
the  two  accessories  in  the  latter  case  are  visible  as  "  ehromal  in  nucleoli  ' 
in  the  resting  stage.  This  difference  in  the  gametes  Guyer  regarded  as 
probably  associated  with  sex-determination.     Gutherz  (1912)  failed  to 


302 


IXTRODFCTIOX  TO  CYTOLOGY 


confirm  ( hiyer's  report  of  a  dimorphism  among  the  sperms,  though  he  also 
observed  the  accessories.  Guyer  in  1914  reasserted  his  conclusions  of 
1912,  but  added  thai  he  was  finding  a  much  larger  chromosome  number 
in  the  cells  of  the  white  man. 

Yon  Winiwarter  (1912),  also  working  upon  the  white  man,  found  in 
the  spermatogonia  and  spermatocytes  47  chromosomes,  including  one 
accessory.  Since  this  accessory  passes  undivided  to  one  pole  in  the  first 
mitosis  and  divides  in  the  second,  half  of  the  sperms  receive  23  and  half 
24  chromosomes.  In  the  cells  of  the  female  there  are  48.  From  these 
data  von  Winiwarter  logically  concluded  that  the  egg  has  24  chromosomes, 
and  that  it  develops  into  a  male  when  fertilized  by  a  sperm  with  23 
chromosomes,  and  into  a  female  when  fertilized  by  one  with  24.  Such  a 
large  number  is  found  in  the  white  man  by  Evans1  also,  but  he  finds  48 
chromosomes  rather  than  47  in  the  spermatogonia,  indicating  the  presence 
of  an  XY  pair  as  in  Drosophila. 


M- 


Fig.  141. — Sex-chromosomes  in  man.  A,  primary  spermatocyte,  negro.  B,  same, 
white.  C,  metaphase  of  heterotypic  mitosis,  negro.  D,  interkinesis,  white.  XY,  the 
sex-chromosomes;  P,  plasmosome.      (After  Wieman,  1917.) 


Wieman  (1917),  using  both  negro  and  white  material,  finds  24  chromo- 
somes in  the  spermatogonia,  two  of  them  being  distinguishable  as  an 
unequal  XY  pair  which  remains  condensed  while  the  autosomes  form  the 
reticulum  (Figs.  140,  141).  In  the  spermatocyte  12  pairs  are  evident, 
including  the  XY  pair.  At  the  first  maturation  mitosis  the  11  autosome 
pairs  separate  into  univalents  as  usual,  but  the  X  and  Y  divide  longitud- 
inally; thus  each  daughter  cell  (secondary  spermatocyte)  receives  11  auto- 
somes and  an  XY  pair.  At  the  second  mitosis  the  11  autosomes  divide 
longitudinally  in  the  normal  fashion  and  the  X  and  Y  separate.  As  a 
result  all  of  the  sperms  receive  12  chromosomes :  in  half  of  them  one  of  the 
12  is  the  X  and  in  the  other  half  it  is  the  Y.  Although  for  a  time  it  ap- 
peared that  the  white  man  had  twice  as  many  chromosomes  as  the  negro, 
a  difference  ordinarily  great  enough  to  mark  them  as  distinct  species, 
Wieman  shows  clearly  that  in  his  material  the  two  have  the  same  num- 
ber, and  is  inclined  to  regard  von  Winiwarter's  material  as  in  some  way 
abnormal.  Sex  inheritance  in  man  is  evidently  of  the  XY  type,  as 
Wieman's  researches  and  genetic  data  indicate  with  considerable  clearness ; 
but  why  some  material  should  plainly  show  twice  as  many  chromosomes 

1  Unpublished  work  cited  by  Babcock  and  Clausen,  p.  538. 


SEX  363 

as  other  materia]  is  a  question  which  only  future  investigation  can  answer. 
It  is  not  improbable,  however,  thai  a  segmentation  of  the  chromosomes  at 
points  of  constriction  may  be  mainly  responsible  for  this  condition. 

In  all  of  the  cases  reviewed  above  the  male  produce-  two  sorts  of 
spermatozoa  differing  visibly  in  chromatin  content:  in  the  language  of 
Mendelism,  the  male  is  "heterozygous  for  sex."  The  female  produces 
but  one  kind  of  egg;  she  is  "homozygous  for  sex."  The  sex  of  the  off- 
spring is  clearly  dependent  on  the  kind  of  spermatozoon  which  timet  ion-, 
and  is  therefore  definitely  correlated  with  the  chromosome  mechanism. 

Female  Heterogametic.- -Thwc  are  also  on  record  a  number  of  cases 
chiefly  among  moths  and  birds,  in  which  the  female  produces  two  kinds 
of  eggs  differing  in  chromosome  content,  while  the  male  produces  but  one 
kind  of  spermatozoon:  the  female  is  heterozygous  for  sex,  and  therefore 
heterogametic,  while  the  male  is  homozygous.  Certain  cases  of  this  type 
will  now  be  reviewed. 

In  the  moth,  Phragmatobia,  Seiler  (1913)  has  described  the  following 
condition.  In  the  male  the  somatic  number  of  chromosomes  is  56, 
including  54  autosomes  and  two  Z-chromosomes1  (Figs.  139  F\  110). 
Eaeh  sperm  receives  28  (27  +  Z).  In  the  female  the  somatic  number 
is  likewise  56,  but  includes  a  ZW  pair  instead  of  the  ZZ  pair  of  the 
male.  Half  the  eggs  receive  27  +  Z  and  the  other  half  27  +  Ww  (the 
^-chromosome  breaks  temporarily  into  two  parts  during  maturation). 
An  egg  with  28  (27  +  Z)  chromosomes  fertilized  by  a  sperm  with 
28  (27  +  Z)  develops  into  a  male  moth  with  56  (54  +  ZZ).  An  egg 
with  29  (27  +  Ww)  chromosomes  fertilized  by  a  sperm  with  28  (27  + 
Z)  develops  into  a  female  moth  with  57  (54  +  ZWw).  The  W  and 
w  subsequently  reunite  to  form  a  single  W,  both  sexes  then  having 
the  same  number,  56.  Since  some  embryos  show  more  than  the  normal 
number  of  chromosomes  Seiler  thinks  it  probable  that  the  /-chromo- 
some is  compound  and  may  under  certain  conditions  subdivide  into 
smaller  elements.  The  same  investigator  has  recently  (1919)  reported 
a  digametic  condition  in  the  female  in  two  other  moths.  Taloeporia 
tubulosa  and  Fumea  casta.  In  the  former  the  eggs  have  29  and  30 
chromosomes,  and  in  the  latter  30  and  31. 

In  the  moth,  Abraxas grossulariata  (Doncaster  1914),  sex  inheritance 
is  apparently  of  the  WZ  type,  though  there  is  often  an  aberrant  behavior 
on  the  part  of  the  chromosomes  which  has  not  been  entirely  explained. 

In  the  common  fowls  Guyer  (1909,  1916)  has  made  observations  which 
he  interprets  as  follows  (Fig.  140.).  In  the  male  there  are  IS  chromo- 
somes: 16  autosomes  and  two  accessories.  Both  of  the  latter  go  to  one 
pole  in  the  first  maturation  mitosis,  and  in  the  second  mitosis  they  sepa- 

1  It  is  customary  to  refer  to  the  Bex-chromosomes  in  species  with  sexually  hetero- 
zygous females  as  W  and  /.  instead  of  )'  and  A'  as  in  (lie  more  common  sexually 
heterozygous  males. 


364  INTRODUCTION  TO  CYTOLOGY 

rate.  Half  the  spermatids,  and  therefore  sperms,  receive  nine  chromo- 
somes (8  +  1  accessory),  while  the  other  spermatids  failing  to  receive  an 
accessory  apparently  degenerate.  In  the  female  there  are  17  chromo- 
somes: 16  autosomes  and  one  accessory.  Since  the  accessory  passes 
undivided  to  one  pole  at  the  first  maturation  mitosis  and  divides  at  the 
second,  half  of  the  eggs  receive  nine  chromosomes  (8  +  1  accessory)  and 
half  receive  eight.  An  egg  with  nine  fertilized  by  a  sperm  with  nine 
develops  into  a  male  with  18  (16  +  2  accessories).  An  egg  with  eight 
fertilized  by  a  sperm  with  9  develops  into  a  female  with  17  (16  +  1 
accessory). 

Cases  of  Parthenogenesis. — The  cytological  phenomena  in  those  animals 
reproducing  in  part  by  parthenogenesis  (see  p.  357)  are  of  much  interest 
in  this  connection.  In  the  honey  bee  the  male,  which  develops  from 
an  unfertilized  egg,  has  the  haploid  number  of  chromosomes  in  his  cells, 
whereas  the  female,  arising  from  a  fertilized  egg,  is  diploid.  Similar  in 
some  respects  is  the  case  of  the  gall-fly  (Neuroterus),  in  which  eggs  that 
have  undergone  reduction  develop  into  haploid  males,  while  other  eggs 
are  formed  without  reduction  and  develop  into  diploid  females.  In  the 
male-producing  eggs  of  Phylloxera  there  are  two  sex-chromosomes,  two 
others  being  lost  in  the  polar  body;  in  the  female-producing  egg  all  four 
are  present.  Two  kinds  of  sperms  are  produced,  half  of  them  with  a 
sex-chromosome  and  half  of  them  without  it.  The  latter  kind  degen- 
erate, leaving  only  the  former  functional.  All  eggs,  if  fertilized,  develop 
into  females.  In  Hydatina  senta  those  eggs  producing  one  polar  body 
and  developing  into  females  are  diploid,  whereas  those  giving  off  two 
polar  bodies  and  developing  into  males  are  haploid.  In  all  of  these 
cases  maleness  accompanies  the  haploid,  and  femaleness  the  diploid 
condition1. 

Plants. — Up  to  the  present  time  a  visible  chromosome  difference 
between  the  two  sexes  in  plants  has  been  established  only  in  Sphcero- 
carpos,  the  genus  of  dioecious  liverworts  in  which  Douin  (1909)  and  Stras- 
burger  (1909)  found  two  of  the  spores  of  a  single  tetrad  to  be  male  and  the 
other  two  female.  In  Sphcerocarpos  Donnellii  (Allen  1917,  1919)  (Figs. 
142,  143)  there  are  in  the  cells  of  the  female  gametophyte  seven  autosomes 
which  differ  somewhat  in  length,  and  one  very  large  X-chromosome.  In 
the  cells  of  the  male  gametophyte  there  are  seven  autosomes  and  a  very 
small  F-chromosome.  The  sporophyte  therefore  has  eight  pairs:  seven 
autosome  pairs  and  the  XY  pair.  Although  all  the  stages  in  the  divisions 
at  sporogenesis  have  not  been  seen,  the  evidence  is  sufficient  to  show  that 
the  X  and  Y  separate  in  the  heterotypic  mitosis  and  divide  longitudinally 
in  the  homceotypic.  Two  spores  of  a  tetrad  therefore  receive  an  X- 
chromosome  in  addition  to  the  seven  autosomes;  these  spores  develop 
into  female  gametophytes.     The  other  two  spores  of  the  tetrad  receive 

1  Compare  the  case  of  the  frogs  developing  by  artificial  parthenogenesis,  page  319. 


si-:.\ 


365 


the  Y  in  place  of  i  !,<•  X  and  develop  into  male  gametophytes.  The  same 
condition  has  been  reported  for  Sphoerocarpos  tezanus  (Miss  Schacke 
1919). 

Although  the  situation  in  Sphoerocarpos  suggests  the  XY  type  of  sex 
inheritance  in  Drosophila  and  other  forms,  it  differs  in  several  respects, 


iiiiiii 


Male  gametophyte 


IIIIIII' 


Female  gametophyte 


Fertilized 
egg 


Sporophyte     Sporocyte 


Maturation  divisions 
SPHAE.K0CARPU5 


IIIIIII 


Male  gametophyte 


I  I  Hill' 


Female  gametophyte 


6    &> 


Fig.   142. — The  history  of  the  chromosomes  in   the  life  cycle  of  Sphoerocarpos.        I 

data  of  C.  K.  Allen,  1917,  1919.) 

as  Allen  (1919)  points  out.     In  Sphoerocarpos  the  separation  of  the  XY 

pair  results  in  the  production  of  two  kinds  of  spores  which  develop  directly 

into  haploid  organisms  (gametophytes)  of  two  sexes,  whereas  in  Droso- 

phila  the  corresponding  separation  results  in  two  sorts  of  male  gametes 

which    determine    the    sexes    of    the 

diploid    organisms    developing    from 

the  eggs  they  fertilize.     Furthermore, 

in  animals  wit  h  sex-chromosomes  some 

forms  show  the  presence  of  XX  to  be 

correlated  with  femaleness  and  X  or 

XFwith  maleness  (male  heterozygous 

for   sex),    while   in   other   forms   XX    ™**  :l"d   [TnL^m^An^r    E 
•'»  _     Sphavrocarpos    Doumlhi.         ifter     <  .    /. 

is    correlated    with    maleness    and    X     AUen,  1919.) 

or     XY      with      femaleness      (female 

heterozygous  for  sex1)-     There  is  evidence  to  show  thai  the  K-chromo- 

some  carries  no  sex-factors  in  these  cases,2  though  its  absence  may  result 

in  sterility  (Bridges).     (See  Chapter  XVII.)     In  Sphoerocarpos,  on  the 

1  Sex-chromosomes  referred  to  in  this  case  as  Z  and  W  rather  than  X  and  Y, 

2  £oe  in  this  connection  Castle  1921. 


Fig.    l  13.     ( Jhromosome  groups  from 


366  INTRODUCTION  TO  CYTOLOGY 

oilier  hand,  1  lie*  presence  of  X  is  correlated  with  femaleness,  Y  with 
maleness,  and  XY  with  the  non-sexual  condition  of  the  sporophyte. 
Here  the  Y  is  apparently  as  important  as  the  X  in  the  transmission  of 
sex-factors.  Allen  suggests  that  secondary  sexual  differences  of  the 
gametophytes,  such  as  size,  may  be  connected  with  the  relative  amounts 
of  chromatin  in  the  nuclei:  the  female  gametophyte,  having  the  large 
X-ehromosome  and  therefore  a  distinctly  greater  mass  of  chromatin, 
develops  more  rapidly  and  becomes  much  larger  than  the  male  gameto- 
phyte with  its  small  F-chromosome.  The  primary  sexual  differences 
he  regards  as  due  to  other  factors. 

Although  Sphoerocarpos  affords  the  only  known  example  of  hetero- 
chromosomes  in  plants  it  is  not  improbable  that  other  cases  will  be 
discovered. 

Conclusion. — In  all  of  the  organisms  included  in  the  foregoing  review 
the  individuals  of  the  two  sexes  differ  visibly  in  their  chromosome  com- 
plements. Moreover,  in  most  of  them  the  sexual  differences  are  definitely 
correlated  with  special  distinguishable  chromosomes,  which  are  accord- 
ingly known  as  sex-chromosomes.  The  distribution  of  these  bodies  at 
the  time  of  reduction  results  in  the  production  of  two  kinds  of  male 
gametes  or  two  kinds  of  female  gametes,  and  in  at  least  one  case  two 
kinds  of  spores.  In  all  of  these  the  chromosome  differentiation  in  the 
cells  correponds  to  the  sexual  differentiation  of  the  organisms  into  which 
they  develop.  The  conclusion  appears  unavoidable  that  the  differentia- 
tion of  the  sexes  is  here  determined  by  a  cell  mechanism,  and  that  the  hetero- 
chromosomes have  a  definite  causal  relationship  with  sex.  How  close  this 
relationship  may  be,  and  to  what  degree  it  is  a  fixed  one,  are  as  yet  by  no 
means  clear,  but  it  is  beyond  question  that  the  heterochromosomes  are 
not  the  sole  determining  cause  of  sex,  as  some  workers  have  hastily 
concluded.     To  this  question  we  shall  subsequently  return. 

Sex -chromosomes  and  Mendelism. — The  heterocbromosome  phe- 
nomena are  intimately  bound  up  with  the  whole  matter  of  Mendelian 
inheritance.  According  to  the  Mendelian  interpretation  the  sexes  are 
due,  like  other  heritable  characters,  to  factors  carried  by  the  chromo- 
somes— by  the  heterochromosomes  where  these  are  present.  The  ap- 
proximate 1  : 1  ratio  of  the  sexes  in  most  organisms  is  accounted  for  in 
the  following  manner.  Referring  to  our  typical  Mendelian  pair  of  charac- 
ters in  the  pea,  tall  and  dwarf,  it  is  found  that  when  a  plant  heterozygous 
for  tallness  (Tt)  is  crossed  with  a  pure  recessive  (tt)  the  resulting  off- 
spring are  half  tall  (Tt)  like  one  parent  and  half  dwarf  (tt)  like  the  other, 
a  1  : 1  ratio.  If  it  is  assumed  in  a  similar  manner  that  there  is  a  pair  of 
factors  for  sex,  one  sex  (the  male,  Correns;  the  female,  Bateson)  being- 
heterozygous  and  the  other  a  homozygous  recessive,  a  1  : 1  ratio  of  the 
sexes  results. 

Largely  because  of  the  observed  behavior  of  sex-chromosomes  the 


SEX 


36' 


Mendelian  interpretatioD  has  been  restated  as  follows.  There  is  :i  single 
factor  for  sex.  In  some  organisms  the  presence  of  two  of  these  factor- 
is  correlated  with  femaleness  and  one  with  maleness  (Fig.  111.  A  I:  the 
male,  having  only  one  sex-factor  (S),  is  heterozygous  and  produces 
gametes  of  two  kinds,  with  and  without  the  fact  or;  the  female,  having  two 
sex-factors,  is  homozygous  and  produces  eggs  of  one  kind,  with  one  sex- 
factor  each.  Two  types  of  union  are  here  possible,  giving  males  and 
females  with  one  and  two  sex-factors  respectively.  This  interpretation 
is  directly  applicable  to  those  cases  in  which  the  male  has  one  sex-chromo- 
some and  the  female  two  (Ancyracanthus,  Ascaris),  and  also  to  those 
having  an  XY  pair  in  the  male  and  an  XX  pair  in  the  female  (Droso- 
phila,  Lygceus).     Each  sex-factor  is  thus  thought  to  be  located  in  an  A- 


O0O0 

\    X    / 


OQ 


/  \     /  \ 

OQD0 


Fig.    144.  —  Mendelian  interpretations  of  sex-inheritance.      Explanation  in    text. 

chromosome.  In  other  organisms  (moths  and  birds)  these  conditions 
are  reversed  (Fig.  144,  C),  the  presence  of  two  sex-factors  being  correlated 
with  maleness  and  one  with  femaleness.  The  female  is  thus  heterozygous 
and  produces  eggs  of  two  kinds,  with  and  without  the  factor.  In  the 
next  generation  the  male  receives  two  sex-factors  (in  the  Z-chromosomes 
of  the  egg  and  sperm)  and  the  female  one  (in  the  Z-chromosome  of  the 
sperm;  the  W-chromosome  of  the  egg  carries  no  sex-factors).  In  view  of 
these  two  contrasted  conditions  as  regards  the  quantitative  relationship 
between  factors  and  sex,  it  is  probable  that  the  sex-factors  carried  by  the 
X-chromosomes  are  in  some  manner  different  from  those  in  the  /-chromo- 
somes. It  has  been  suggested  that  in  some  cases  (  Fig.  144,  B)  the  male 
may  have  no  sex-factors  at  all,  the  heterozygous  female  thus  having  one 
more  sex-factor  than  the  male,  as  in  the  homozygous  females  of  Fig. 
144,  A. 

Experimental  Alteration  of  the  Sex  Ratio.     Although  most  organisms 
approximate   closely  the    I  :  1    sex    ratio  called    for   on    the   basis   of   the 


368  INTRODUCTION  TO  CYTOLOGY 

Mendelian  theory  of  sex,  constant  deviations  from  this  ratio  are  fre- 
quently found.  Still  more  significant  is  the  fact  that  in  many  cases  the 
ratio  can  be  markedly  altered  by  changing  the  environmental  condi- 
tions. Thus  R.  Hertwig  (1906,  1912)  and  Kuschakewitsch  (1910)  found 
that  if  the  eggs  of  the  frog  are  allowed  to  become  over-ripe  before 
fertilization,  in  which  case  they  take  up  an  abnormal  amount  of  water, 
the  resulting  individuals  show  an  unusually  high  percentage  (even  100 
per  cent)  of  males.  Conversely,  Miss  King  (1907-1912)  lowered  the 
water  content  of  toad  eggs,  and  with  a  mortality  of  only  a  little  over  6 
per  cent  obtained  80  per  cent  females.- 

In  Dinophilus,  as  already  noted,  there  are  two  kinds  of  eggs  laid: 
large  ones  developing  into  females  and  small  ones  developing  into  males. 
Malsen  (1906)  fouDcl  that  by  altering  the  temperature  the  relative  pro- 
portion of  the  two  sexes  could  be  changed,  but  this  effect  was  brought 
about  through  an  influence  on  the  laying  of  the  eggs:  both  kinds  were 
produced  as  usual,  but  the  laying  of  one  kind  was  hindered. 

The  rotifer,  Hydatina  senta  (Whitney  1914,  1916),  if  scantily  fed  on 
Polytoma,  continues  to  produce  generations  of  parthenogenetic  females, 
but  when  copiously  fed  on  Euglena  females  appear  which  lay  male-pro- 
ducing eggs,  and  sexual  reproduction  then  occurs.  According  to  Shull 
and  LadofT  (1916)  the  percentage  of  males  is  here  correlated  with  the 
supply  of  oxygen  which  counteracts  certain  agencies  (accumulated 
substances  in  the  water)  tending  to  decrease  male  production.  Whitney 
(1919),  however,  contends  that  oxygen  is  not  a  factor  affecting  sex  in 
Hydatina. 

In  interpreting  such  results  as  these  considerable  care  should  be 
exercised  in  distinguishing  an  actual  determination  of  the  sex  of  an 
individual  from  a  number  of  other  phenomena  which,  though  they  may 
appear  like  sex-determination,  are  not  to  be  regarded  as  such  in  the 
strict  sense.  In  many  cases  in  which  changed  environmental  factors 
have  been  shown  to  have  an  influence  on  the  sex  ratio  it  is  clear  that  the 
results  are  not  due  to  an  actual  reversal  or  determination  of  the  sex  in 
any  individual,  but  rather  to  the  fact  that  the  new  conditions  imposed 
have  caused  a  greater  mortality  among  the  eggs  or  embryos  of  one  sex,  so 
that  those  of  the  other  sex  preponderate.  Although  the  ratio  of  the  sexes 
may  here  be  subject  to  an  experimental  control,  the  sex  of  no  given 
individual  is  actually  determined  or  altered. 

Indirect  control  of  another  type  is  seen  in  organisms  whose  sex  is 
dependent  upon  the  form  of  reproduction  (zygogenetic  or  partheno- 
genetic). Environmental  conditions  may  in  such  cases  influence  the  form 
of  reproduction  resorted  to,  and  therefore  the  sex  of  the  animals  resulting; 
but  the  sex  of  no  individual,  once  started,  is  altered.  Morgan  points  out 
that  the  change  of  diet  in  Hydatina,  instead  of  altering  sex  directly, 
induces  the  formation  of  a  new  type  of  female  which  may  either  function 


SEX  369 

sexually    or    produce    eggs    which     develop    parthenogenetically    into 

males. 

Another  case  in  point  is  that  of  Dinophilus,  in  which  abnormal  temper- 
ture  conditions  prevenl  the  laying  and  development  of  eggs  from  which 

the  individuals  of  one  sex  normally  arise.     As  Thomson  (  L913,  p.  502 
remarks,    ".    .    .if   nutritive   and   other   environmental    influences   .in- 
operative, it  is,  in  the  main,  by  affecting  the  production  and  survival  of 
sexually-predestined  germ  cells." 

A  clear  case  of  sex-determination  in  the  strict  sense  would  be  one  in 
which  a  given  parthenogenetic  egg,  fertilized  egg,  spore,  or  young  individ- 
ual, in  a  unisexual  organism,  could  be  made  to  develop  at  will  into  either 
sex;or  in  which  such  an  organism,  already  showing  the  essenl  ial  characters 
of  one. sex,  could  be  made  to  develop  into  the  other  sex  (sex-reversal). 
Evidence  that  in  certain  cases  such  a  determination  or  reversal  can 
actually  be  accomplished  has  had  much  to  do  with  the  development  of 
recent  metabolic  theories  of  sex. 

Metabolic  Theories  of  Sex — Animals. — According  to  the  metabolic 
theories  of  sex,  the  difference  between  the  two  sexes  is  primarily  one  oi 
metabolism,  the  difference  being  not  necessarily  in  the  kind  of  metabolic 
processes,  as  Geddes  and  Thomson  thought,  but  more  probably  in  their 
rate  or  level.  One  of  the  most  prominent  exponents  of  this  type  of 
theory  is  Riddle  (1912,  etc.),  who  has  continued  the  important  researches 
on  pigeons  begun  by  Whitman  many  years  ago.  At  each  laying  these 
birds  produce  a  pair,  or  clutch,  of  eggs.  Under  normal  condition-  the 
first  egg  laid  develops  into  a  male  and  the  second  into  a  female.  By  a 
careful  analysis  of  a  large  number  of  eggs  Riddle  has  been  able  to  show 
that  the  yolk  of  the  first  egg  is  the  smaller,  and  has  the  smaller  amount  of 
storage  material  (fats  and  phosphorus-bearing  compounds),  the  higher 
water  content,  and  the  higher  oxidizing  capacity,  or  higher  metabolism. 
The  second  or  female-producing  egg  therefore  differs  in  having  a  Larger 
yolk,  more  storage  material,  less  water,  and  a  lower  metabolism.  The 
chromosomes  of  the  pigeon  are  not  accurately  known,  but  it  is  probable, 
as  indicated  by  the  behavior  of  the  sex-linked  factors  (see  next  chapter), 
that  the  WZ  type  of  sex-chromosome  differentiation  found  in  other  bird- 
is  present  here.  Whatever  their  cytological  differences  may  be.  Ih<  two 
sexes  are  characterized  in  the  egg  stage  by  different  levels  of  metabolism^  tin 
male  having  the  higher  level  and  thefemaU  the  lower;  and  this  difference  has 
been  shown  (as  indicated  by  the  metabolism  of  the  blood)  to  persist  into 
the  adult  stage. 

The  seasonal  change  in  the  amount  of  storage,  and  consequently  in  the 
level  of  metabolism,  is  definitely  con-elated  with  the  percentage  of  a 
given  sex  and  the  degree  of  it>  manifestation.  As  the  season  advances 
both  eggs  of  the  clutch  -tore  more  energy-cunt aining  materials,  and  the 
birds  developing  from   the  second   (female-producing)  egg,   while  often 

24 


370  INTRODUCTION  TO  CYTOLOGY 

quite  "masculine,:  in  their  secondary  sex  characters  early  in  the  season, 
become  increasingly  "  feminine  "  toward  the  end  of  the  season.  Moreover, 
the  total  percentage  of  females  increases.  By  influencing  storage,  water 
content,  and  the  general  metabolic  condition  of  these  eggs  Riddle  has 
been  able  to  induce  experimentally  an  actual  reversal  of  their  natural  sex 
tendencies.  Hence  he  concludes  that  sex  is  a  quantitative,  modifiable, 
and  "fluid"  character;  the  two  sexes  do  not  represent  two  qualitatively 
distinct,  mutually  exclusive  properties,  but  are  rather  two  conditions  of 
one  general  property — two  levels  in  a  continuous  series  of  metabolic 
states  passing  gradually  one  into  another.  Accordingly,  if  the  two  levels 
normally  maintained  can  be  sufficiently  altered  a  series  of  intergrades 
between  the  two  sexes  and  an  alteration  of  the  sex  itself  should  be  possible, 
and  this  Riddle  has  apparently  accomplished. 

On  the  basis  of  this  metabolic  theory  of  sex  Riddle  interprets  the 
results  obtained  by  Miss  King  with  toad  eggs,  by  Hertwig  with  those  of 
t  he  frog,  by  Whitney  and  Shull  with  rotifers,  and  by  other  investigators 
to  be  mentioned  below.  The  correlation  of  high  and  low  water  content 
with  maleness  and  femaleness  respectively  in  toads  and  frogs,  and  that  of 
change  of  food  and  increased  oxygen  supply  with  maleness  in  rotifers,  are 
held  to  be  in  harmony  with  similar  correlations  which  have  been  shown  to 
exist  in  pigeons. 

The  theory  that  sex  is  a  quantitative,  reversible  state  closely  asso- 
ciated with  metabolic  conditions  is  strongly  supported  by  the  researches  of 
Goldschmidt  (1916a6,  1917),  Banta  (1916),  and  Lillie  (1917).  In  the 
gypsy  moth,  Lymantria  dispar,  which  is  cytologically  heterogametic, 
Goldschmidt  has  been  able  to  induce  experimentally  a  large  series  of  "sex 
intergrades  "  between  the  male  and  female  conditions.  It  appears  that  an 
individual  of  either  sex,  after  beginning  its  development,  may  be  made  to 
develop  the  characters  of  the  other  sex  partially  or  completely,  the  degree 
of  alteration  depending  upon  the  time  at  which  the  change  sets  in.  Gold- 
schmidt concludes  that  normal  individuals  of  both  sexes  must  have  both 
sex  capabilities,  the  sex  manifested  depending  upon  the  relative  strength 
with  which  they  are  caused  to  act  by  certain  conditions.  He  thinks  it  prob- 
able that  the  hereditary  characters  have  their  material  basis  in  enzymes 
or  substances  of  a  similar  nature,  those  associated  with  femaleness  and 
maleness  being  termed  "  gynase  "  and  "  andrase  "  respectively.  Although 
for  a  time  he  interpreted  the  behavior  of  the  sexes  in  Lymantria  on  the  basis 
of  Mendelian  factors,  he  is  now  inclined  to  view  this  form  of  explanation 
as  inadequate.  The  chromosome  behavior  in  these  moths  is  not  well 
known,  but  breeding  experiments  with  an  intersexual  female  functioning 
as  a  male  show  the  results  which  would  be  expected  on  the  assumption 
thai  she  had  the  PTZ-chromosome  constitution.  This  would  mean  that 
the  moth  in  question  had  had  its  sex  reversed  without  any  visible 
change  in  its  chromosome1  complement,  but  this  intrepretation  awaits 
cytological  confirmation. 


SEX  ;;;, 

In  the  phyllopod  crustacean,  Simocepkalus  vetulus,  Banta  1 1916)  finds 
that  under  certain  experimental  conditions".    .    .  the  same  individual 

even  the  same  sex  gland,  may  develop  eggs  and  sperms  at  1  lie  same  time  or 

sperms  at  one  time  and  eggs  at  another  time."     Banta  looks  upon 
as  not  a  fixed  or  definite  state  but  rather  "a  purely  relative  thing" 
dependent  upon  the  general  physiological  state  of  the  organism  which  in 
turn  is  under  the  influence  of  environmental  factors.     Be  also  reports 
sex  intergrades  in  Daphnia  longi spina  (1918). 

Lillie  (1917abc),  as  the  result  of  a  study  of  twins  in  cattle,  concludes 
that  "sex-determination  in  mammals  is  zygotic,  but  it  does  not  imply  an 
irreversible  tendency  to  the  indicated  sex-differentiation.  Intensification 
of  the  male  factors  of  the  female  zygote  from  the  time  of  onsel  of  sexual 
differentiation  by  action  of  sex  hormones  may  bring  about  very  exten- 
sive reversal  of  the  indicated  sex-differentiation"  (1917c). 

Intersexes  have  recently  been  described  in  Drosophila  simulant  by 
Sturtevant  (1920).  All  the  intersexual  flies  are  modified  females  and 
show  the  same  grade  of  intersexuality.  Sturtevant  believes  t  he  condit  ion 
to  be  due  to  the  action  of  a  mutant  gene.  If  this  interpretation  is 
applied  generally  to  the  phenomenon  of  intersexuality  it  is  evidenl 
that  certain  genes  at  least  are  modifiable  by  environmental  conditions, 
for  the  reason  that  such  conditions  so  often  evoke  the  intersexual  state. 

A  very  striking  instance  of  the  control  exercised  by  environmental 
factors  over  the  sex  of  a  given  individual  is  found  in  Bonellia  (Baltzer 
1914).  In  this  gephyrean  worm  the  male  is  very  small  and  degenerate, 
and  lives  parasitically  upon  the  long  proboscis  and  in  the  nephridium 
of  the  much  larger  female.  If  young  individuals  are  kept  in  an  aquarium 
by  themselves  they  develop  into  females,  but  if  they  are  placed  with 
mature  females  they  settle  upon  the  probosces  of  the  latter  and  develop 
into  males.  By  allowing  them  to  remain  on  the  probosces  for  varying 
lengths  of  time  Baltzer  was  able  to  obtain  many  intergrades  between  the 
typical  male  and  female  conditions.  It  is  plain  that  the  proboscis 
exercises  an  influence  over  the  sex  of  the  young  animal,  but  whether  this 
is  through  a  secretion  or  some  other  agency  is  not  known. 

A  somewhat  similar  case  is  that  of  Crepidula  plana,  described  by 
Gould  (1917).  This  mollusk  is  hermaphroditic  but  completely  protan- 
dric,  i.e.,  the  male  and  female  phases  are  entirely  separate  in  time,  the 
former  developing  first.  The  development  of  the  male  phase  is  dependent 
upon  the  presence  of  a  larger  individual  (not  necessarily  a  female)  of  the 
same  species.  If  a  male  is  removed  from  the  neighborhood  of  a  Larger 
individual  the  male  organs  degenerate,  and  after  a  period  of  sexual 
inactivity  the  animal  becomes  a  female.  The  nature  of  the  stimulus 
exerted  by  the  larger  individual  has  not  been  ascertained. 

Plants-  Experiments  of  a  corresponding  nature  on  sex  modification 
in  plants  are  on  the  whole  less  conclusive  than  those  on  animals,  for  the 


372  INTRODUCTION  TO  CYTOLOGY 

reason  that  many  of  them  have  been  carried  out  with  angiosperms,  in 
which  hermaphroditism  is  so  common  and  which  are  regarded  by  some 
botanists  as  all  potentially  bisexual.  As  a  case  illustrating  the  latter 
point  may  be  cited  the  hemp  plant,  Cannabis  sativa.  This  species  is 
normally  unisexual,  but  if  the  flowers  are  removed  it  will  produce  flowers 
of  the  other  sex  (Prit chard  1916). 

Intersexes  of  many  grades  between  the  normal  plants  in  dioecious 
angiosperms  are  frequently  found,  as  in  Myrica  Gale  (Davey  and  Gibson 
1917),  Cannabis  sativa,  Salix  amygdaloides,  and  Morus  alba  (Schaffner 
1919ab),  whereas  the  relative  proportions  of  maleness  and  femaleness  in 
hermaphroditic  forms  are  apparently  very  easily  influenced  by  the 
environment.  In  Plantago  lanceolata  (Bartlett  1911;  Correns  1908; 
Stout  1919)  the  stamens  and  pollen  are  developed  in  various  degrees  in 
different  flowers,  or  even  in  the  same  flower;  in  some  cases  they  are  only 
rudimentary,  leaving  the  flowers  functionally  female.  Stout  inclines 
to  the  view  that  dicecism  results  from  the  suppression  of  femaleness  or 
maleness  in  organisms  originally  monoecious,  and  points  out  that  the 
many  " degrees  of  maleness"  in  Plantago  fill  the  gap  completely  in  this 
case.  He  accordingly  concludes,  in  agreement  with  Riddle,  Banta, 
and  Goldschmidt,  that  sex  is  a  labile,  reversible  character;  that  maleness 
and  femaleness,  both  present  in  the  somatic  cells  of  all  sporophytic 
individuals,  are  relative  and  not  absolute  conditions;  and  that  " sex- 
determination,  at  least  in  hermaphrodites,  is  fundamentally  a  phe- 
nomenon of  somatic  differentiation  that  is  ultimately  associated  with 
processes  of  growth,  development,  and  interaction  of  tissues,  and  subject 
to  modification  or  even  complete  determination  by  them."  The  sex- 
chromosome  theory  is  held  to  be  inadequate  in  the  case  of  hermaphro- 
dites: sex  is  an  "epigenetic,"  not  a  " preformed"  character. 

Yampolsky  (1919,  1920)  thinks  it  probable  that  male  and  female 
gametes  with  graded  potencies  are  produced  in  Mercurialis  annua,  and 
that  the  sexes  cannot  be  due  to  a  segregation  of  sex-factors  at  reduction. 

The  sex  of  the  haploid  phase  (gametophyte)  of  the  plant  life  cycle, 
when  this  phase  is  unisexual  (dioecious),  behaves  as  a  much  more  nearly 
irreversible  character,  as  shown  by  the  experiments  on  mosses,  Marchantia, 
and  Phycomyces  cited  early  in  the  present  chapter.  How  widely  this  holds 
true  in  thallophytes  and  bryophytes  is  not  known.  Allen  regards 
the  quantitative  theory  of  sex  as  quite  inapplicable  to  the  case  of 
Sphcerocarpos.  In  the  homosporous  pteridophytes  the  gametophytes  are 
commonly  monoecious,  and  the  fact  that  here,  as  in  many  liverworts, 
the  male  organs  appear  early  and  the  female  organs  later  suggests  a 
physiological  basis  of  sex-determination.  Some  fern  gametophytes,  on 
the  contrary,  are  dioecious  under  all  ordinary  conditions.  In  Onoclea 
(Wuist  1913),  such  a  normally  dioecious  form,  the  female  gametophytes 
under  certain  experimental  conditions  produced  antheridia  in  addition 


SEX  373 

to  their  archegonia,  but  the  male  gametophytes  could  n« >i  be  made  to 
develop  archegonia.  In  Osmunda  regalis  japonica  and  Asplenium  nidus 
which  are  monoecious,  Nagai  (1915)  found  thai  the  concentratioa  of  the 
Knop's  nutrient  solution  used  has  a  controlling  influence  over  the  kind 
of  sex  organ  appearing,  the  number  of  antheridia  in  general  decreasing 
with  the  concentration. 

These  results,  taken  together  with  the  fact  that  many  gametophyto 
especially  those  of  homosporous  pteridophytes.  are  monoecious,  -how  thai 
the  capabilities  of  both  sexes  can  be  present  in  the  haploid  nucleus   as 
Strasburger  thought,  although  in  many  forms  the  visibly  developed  sexes 
may  be  automatically  determined  by  some  cell  mechanism. 

General  Discussion, — We  have  now  reviewed  some  of  the  evidenc 
which  have  led  to  two  general  theories  of  sex  and  its  determination.     (  me 
theory  represents  an  attempt  to  account  for  the  phenomena  in  question 
on  the  basis  of  a  morphological  cell  mechanism  whereby  Mendelian  or 
other  factors  are  distributed  in  a  definite  and  fixed  manner,  whereas  on 
the  other  theory  it  is  held  that  they  are  the  results  of  a  physiological 
differentiation  manifesting  itself  chiefly  in  alterable  levels  of  metabolism. 
Although  these  two  conceptions  may  appear  to  be   mutually  exclusive 
if  expressed  in  too  uncompromising  a  form,  both  must  contain  elements 
of  truth.     It  is  beyond  question  that  the  two  manifestations  of  sexual 
differentiation,   the   physiological  and   the   morphological,   are  both   of 
importance   and   cannot   be   ultimately   irreconcilable:   our   task    i-    to 
determine  their  relative  significance  and  to  discover  the   nature  and 
degree  of  their  mutual  interdependence.     It  seems  clear  that  the  digam- 
etic  condition  when  present  in  dioecious  forms  does  regulate  the  ratio 
of  the  sexes:  under  all  ordinary  circumstances  the  sex  of  the  individual 
is  here  dependent  upon  the  kind  of  gamete  which  gives  rise  to  it  (or  the 
kind  of  spore  in  the  case  of  certain  gametophytes).     Hut    it    i>  to  be 
emphasized  that  the  dimorphism  shown  by  such  gametes  or  spores  i-  not 
entirely  a  morphological  one,  or  even  mainly  so:  in  many  cases  no  morpho- 
logical difference  can  be  detected  although  the  two  are  clearly  different  in 
physiological   behavior,   as   shown   by   the    spores    of  Phycomyces  and 
certain   bryophytes.     It    is  generally  inferred    that    here   a    structural 
difference,  although  invisible,  is  nevertheless  present. 

In  this  connection  it  may  be  recalled  that  the  differences  between  the 
male  and  female  gametes,  irrespective  of  any  different  ial  ion  which  may  !"• 
present  among  those  of  either  kind,  are  both  physiological  and  morpho- 
logical. The  primary  characters  <>t"  sex  are  those  possessed  by  the 
gametes  themselves,  and  the  principal  distinction  between  the  male  and 
female  gametes  seems  to  be  a  physiological  one  which  is  manifested  in 
their  mutual  attraction  and  fusion.  Any  visible  morphological  differ- 
entiations that  they  may  possess  an1  to  be  regarded  as  secondary  adapta- 
tions to  unlike  functions,  because  of  the  fact  thai  in  many  of  the  lower 


374  INTRODUCTION  TO  CYTOLOGY 

organisms  there  is  no  discernible  structural  difference  between  the 
male  and  female  gametes,  and  further  because  structural  differences 
may  be  annulled  in  certain  instances  (some  gregarines).  Any  material 
differences  present  in  visibly  similar  gametes  are  more  probably  chemical 
in  nature,  the  structural  difference  being  of  a  molecular  order.  Indeed, 
there  is  probably  no  physiological  difference  without  a  structural  differ- 
ence of  this  sort.  If  the  term  structure  be  extended  to  include  molecular 
constitution  the  discussion  over  the  relative  priority  of  structural  and 
physiological  differentiation  becomes  futile,  for  at  this  level  the  two  are 
aspects  of  one  and  the  same  change.  It  is  only  when  we  restrict  the  term 
structure  to  the  grosser,  visible  features  that  we  can  speak  of  physiological 
differentiation  as  preceding  alteration  in  structure.  Ultimately  structural 
and  functional  changes  are  indistinguishable.  Just  as  in  the  gametes 
of  the  two  sexes,  so  also  in  the  unisexual  individuals  which  the  gametes 
produce  there  may  be  striking  differences  of  both  morphological  and 
physiological  natures;  but  if  we  use  the  term  morphological  only  with 
reference  to  visible  features  the  primary  distinction  between  the  sexes 
in  organisms  of  all  grades  is  apparently  one  of  physiological  state,  this 
distinction  and  its  result  (sexual  reproduction)  being  of  the  greatest 
biological  importance. 

Taking  into  consideration  all  organisms,  low  and  high,  it  seems 
probable  that  any  dimorphism  among  the  gametes  of  one  sex  or  the  other 
has  in  some  way  been  developed  in  connection  with  the  maintenance  of 
the  above  mentioned  difference  in  physiological  state  in  organisms  of  a 
certain  level  of  advancement.  Different  organisms  show  all  degrees  in 
the  differentiation  among  the  gametes  of  one  sex:  some  are  marked  by  an 
absence  of  any  visible  difference  either  in  the  gametes  or  in  the  her- 
maphroditic individuals  produced;  in  others  the  gametes  (of  one  sex)  are 
visibly  similar  but  result  in  male  and  female  individuals  in  regular  ratio; 
and  finally  there  are  those  in  which  the  gametes  are  of  two  kinds  both 
physiologically  and  morphologically,  the  two  kinds  controlling  the  pro- 
duction of  individuals  of  the  two  sexes.  It  is  in  organisms  of  the  last  type 
especially  that  the  question  of  sex-determination  finds  the  adherents  of 
the  chromosome-Mendelian  theory  and  those  of  the  metabolic  theory 
in  disagreement.  Is  the  sex  of  the  individual  inevitably  dependent 
upon  the  type  of  gamete  functioning  (usually  the  kind  of  sperm  fer- 
tilizing the  egg),  or  is  it  possible  to  overcome  the  effect  which  the 
chromosome  mechanism  may  have  by  influencing  sufficiently  the 
metabolism  of  the  organism?  If  the  sex  of  an  individual  is  so  changed, 
does  the  chromosome  mechanism  undergo  a  corresponding  alteration? 

Those  who  have  developed  the  chromosome-Mendelian  theory  have 
perhaps  too  often  held  that  the  two  sexual  states,  maleness  and  female- 
ness,  are  in  their  ultimate  analysis  mutually  exclusive — that  they  are  two 
fundamental  and  qualitatively  different  alternative  characters  depending 


SEX  375 

unalterably  upon  unit  factors.  The  hereditary  factors  m  genes  are 
usually  regarded  by  geneticists  as  unmodifiable  excepl  by  sudden  muta- 
tions. Failure  to  distinguish  between  (he  modification  of  genes  and  ill- 
modification  of  the  interaction  of  genes  during  ontogenesis  has  led  many 
to  the  view  that  the  sex  of  the  individual  must  be  rigidly  fixed  al  fertiliza- 
tion in  digametic  and  dioecious  forms,  or  at  reduction  in  the  case  of  dioe- 
cious gametophytes  like  those  of  Sphcerocar pos.  It  is  very  difficult  to 
reconcile  any  inflexible  theory  of  this  nature  with  the  great  diversity  of 
situations  known  without  resorting  to  hypotheses  of  somatic  segregation 
of  factors,  alterations  in  dominance,  and  other  assumptions  not  well 
supported  by  observational  evidence.  Such  difficulties  are  encountered 
in  the  common  hermaphroditic  condition  of  gametophytes,  which  are 
haploid;  in  the  possibility  of  causing  the  development  of  the  second 
sex  in  gametophytes  normally  unisexual  (Onoclea);  and  especially  in  the 
numerous  cases  of  sex  intergrades,  in  which  it  is  possible  not  only  to  con- 
trol the  relative  amounts  of  maleness  and  femaleness  in  hermaphroditic 
forms,  but  also  to  produce  all  intermediate  grades  between  male  and 
female  individuals,  and  furthermore  to  reverse  the  sex  in  unisexual  forms, 
even  in  certain  species  with  sex-chromosomes  (moths  and  probably 
pigeons). 

If,  in  accordance  with  the  ideas  of  many  biologists,  sex  is  held  to 
be  a  quantitative,  " fluid"  character  associated  with  a  continuous  series 
of  physiological  states  which  may  pass  into  one  another,  the  way  i-  open 
for  the  explanation  on  a  common  basis  of  all  cases  of  hermaphrodit  ism  in 
both  haploid  and  diploid  individuals,  of  sex  intergrades,  and  of  the  experi- 
mental modification  of  sex.  At  the  same  time  the  influence  of  the  sex- 
chromosomes  or  even  of  smaller  factors  within  them  may  be  allowed. 
As  Riddle  (1917)  states,  organisms  have  had  the  problem  of  producing 
germs  of  two  metabolic  levels,  and  in  some  cases  this  has  led  to  the 
establishment  in  the  two  sexes  of  two  amounts  of  chromat  in  or  even  of  t  \\  <> 
different  chromosome  complements.  The  sex-chromosomes,  or  units 
contained  within  them,  act  with  others  in  the  maintenance  of  two 
diverse  levels  of  metabolism  in  the  gametes  and  in  the  offspring,  and  with 
these  levels  are  correlated  the  two  conditions  which  we  distinguish  as 
male  and  female.  Even  if  the  sexes  in  such  cases  do  not  differ  in  the 
quality  of  their  chromatin,  they  at  least  differ  quantitatively  in  this 
respect,  in  agreement  with  the  theory  that  sex  isaquantitat  ive character. 
The  chromosome  difference  being  only  one  factor  in  .1  complex  system 
producing  the  two  sexual  states,  and  no  single  element  in  tin-  System 
being  the  sole  determiner  of  sex,  it  is  not  impossible  that  the  r\)\'r\  of  this 
one  factor  should  be  annulled  by  sufficiently  altering  the  other  factors 
and  thus  modifying  the  action  of  the  factorial  system  as  a  whole.  Tin- 
same  is  to  be  said  of  other  characters  also.  What  an  organism  inherits 
is  not   simply  this  or  that    character  or  sex,   but    rather  a    tendency   to 


376  INTRODUCTION  TO  CYTOLOGY 

develop  a  definite  group  of  characters,  including-  a  particular  sex,  under 
a  given  set  of  environmental  conditions. 

In  those  organisms  possessing  heterochromosomes  the  sex  of  the 
individual  under  all  ordinary  circumstances  is  dependent  upon  the  kind 
of  sperm  (or,  in  some  cases,  the  kind  of  egg)  functioning  at  fertilization, 
and  does  not  change  thereafter.  Furthermore,  it  apparently  cannot  be 
changed  by  many  methods  commonly  supposed  to  be  efficacious  in  this 
respect.  So  far  the  chromosome  theory  is  valid;  but  it  does  not  follow 
from  this  that  the  sex  which  is  characterized  by  a  certain  physiological 
state  and  is  correlated  with  a  certain  type  of  chromosome  complement 
and  a  variety  of  secondary  sexual  characters,  is  so  firmly  fixed  that  it 
cannot  be  altered  by  any  extraordinary  means.  The  metabolic  state, 
even  though  its  regulation  may  be  accomplished  in  part  through  a  visible 
mechanism,  is  the  resultant  of  a  complex  series  of  reactions  which  may  be 
interfered  with  at  many  points.  In  some  cases  this  metabolic  state  has 
been  artificially  altered  to  a  degree  sufficient  to  bring  about  an  actual 
reversal  of  the  sex.  It  is  admitted  that  other  heritable  characters  de- 
finitely associated  with  constant  genes  are  greatly  modified  in  the  manner 
and  degree  of  their  expression  by  environmental  influences,  and  the 
evidence  now  at  hand  indicates  that  no  exception  to  this  rule  can  be  made 
in  the  case  of  sex. 

In  criticizing  the  results  and  interpretations  of  Riddle,  Morgan  (1919a) 
points  out  that  the  behavior  of  a  certain  sex-linked  character  worked  out 
by  Strong  (1912)  indicates  that  the  females  which  were  "  changed  into 
males"  have  the  male  chromosome  complement,  and  that  sex  is  as  much 
a  matter  of  chromosomes  here  as  elsewhere.  He  declares  further  that 
there  is  as  yet  no  known  case  in  which  the  sex  determined  by  a  chromo- 
some mechanism  has  been  changed  by  other  agencies  in  spite  of  the 
chromosome  arrangement.  The  evidence  here  points  to  the  conclusion 
that  when  an  alteration  of  the  sex  is  induced,  this  does  not  occur  with- 
out a  corresponding  alteration  in  the  chromosome  mechanism.  In  Droso- 
phila,  however,  Sturtevant  (1920)  finds  that  the  intersexes  observed  by 
him  are  modified  females  with  the  usual  two  X-chromosomes.  Here, 
therefore,  certain  male  characters  at  least  are  present  in  an  organism  with 
the  female  chromosome  complement. 

The  number  of  instances  of  change  of  sex  in  forms  normally  controlled 
by  a  chromosome  mechanism  will  probably  increase  as  the  nature  of 
sexual  differentiation  becomes  more  fully  known  and  as  experimental 
technique  improves;  but  it  is  also  probable  that  in  animals  the  sex  of 
which  we  are  most  desirous  of  controlling,  practical  difficulties  may  pre- 
vent the  attainment  of  satisfactory  results.  Slight  differences  in  the 
responses  of  the  two  kinds  of  male  gametes  (or  female  gametes)  might 
conceivably  make  possible  a  control  over  the  kind  functioning,  but  it 
seems  more  probable  that  the  sex  of  the  individual  will  be  found  to  be 


sex  377 

more  easily  influenced,  it"  al  all,  after  fertilization.  Bui  ii  is  quite  un- 
known whether  or  not  an  individual  which  had  undergone  a  reversal  of 
sex  would  be  as  successful  biologically  as  a  normal  one.  Unusual  fea- 
tures may  be  expected  in  a  sexually  fund  ional  organism  wit  h  I  be  chromo- 
some complement  normally  accompanying  the  other  sex.  it  such  an 
organism  should  be  found;  but  such  interesting  questions  can  only  be 
answered  by  the  facts  yielded  by  further  research. 

All  questions  of  sex  control  are  secondary  to  the  main  problem  of  the 
ultimate  nature  of  sex,  a  problem  which  reveals  itself  with  increasing 
clearness  as  primarily  a  physiological  one.  The  question  of  the  origin 
and  significance  of  sex  is  one  which  lies  outside  the  scope  of  this  work. 

Bibliography  at  end  of  Chapter  XVIII. 


CHAPTER  XVII 
LINKAGE 

In  Chapter  XV  attention  was  directed  to  the  remarkable  parallelism 
which  exists  between  the  distribution  of  the  Mendelian  characters  and 
that  of  the  chromosomes.  A  vast  number  of  breeding  experiments  with 
both  plants  and  animals  have  shown  that  new  combinations  of  characters 
are  formed  at  the  time  of  fertilization,  when  two  parental  sets  of  chromo- 
somes are  brought  together,  and  that  a  segregation  of  characters  occurs 
at  the  time  of  reduction,  when  the  chromosomes  are  sorted  out  into  two 
groups.  Moreover,  the  distribution  of  a  single  allelomorphic  pair  of 
Mendelian  characters  parallels  precisely  that  of  a  single  homologous  pair 
of  chromosomes.  These  facts  indicate  clearly  that  chromosomes  and 
characters  are  in  some  manner  causally  related.  This  conclusion  is 
strongly  supported  by  the  cytological  aspects  of  sex  inheritance,  maleness 
and  femaleness  in  a  large  number  of  reported  cases  being  definitely  corre- 
lated with  the  activity  of  certain  distinguishable  chromosomes. 

It  has  also  been  pointed  out  that  the  hypothesis  upon  which  these 
phenomena  are  generally  interpreted  is  that  the  characters  are  repre- 
sented in  the  chromosomes  by  material  factors,  or  genes,  which  in  some 
way  control  the  development  of  characters  in  the  individual.  Since,  now, 
an  organism  usually  has  many  more  Mendelian  character  pairs  than  it  has 
chromosome  pairs,  one  pair  of  chromosomes  must  as  a  rule  carry  genes 
for  more  than  one  pair  of  characters.  Furthermore,  the  different  pairs 
of  chromosomes  are  entirely  independent  of  one  another  in  distribution. 
It  would  therefore  follow  that  if  two  allelomorphic  character  pairs  have 
their  genes  located  in  different  chromosome  pairs,  they  will  be  quite 
independent  of  each  other  in  their  inheritance  through  a  series  of  genera- 
tions; whereas,  if  their  genes  are  located  in  the  same  pair  of  chromo- 
somes, they  will  be  inherited  together.  The  latter  condition — the 
persistent  association  of  characters  belonging  to  different  allelomorphic 
pairs  through  a  series  of  generations — actually  exists  and  is  known  as 
linkage. 

The  phenomenon  of  linkage  was  discovered  in  1906  by  Bateson  and 
Punnett  in  the  sweet  pea.  They  found  flower  color  to  be  linked  with  the 
shape  of  the  pollen  grain:  purple  flowers  nearly  always  had  long  grains, 
while  red  flowers  had  round  grains.  The  possible  relationship  between 
linkage  and  the  chromosome  hypothesis  was  pointed  out  by  Lock  in  the 
same  year.     Linkage  relations  have  since  been  worked  out  in  a  consider- 

378 


LINKAGE 


379 


able  number  of  plants  and  animals,  and  are  especially  well  known  in 
the  case  of  the  fruit,  fly,  Drosophila  melanogaster,  owing  to  the  exhaustive 
researches  of  Morgan  and  his  coworkers. 

A  Typical  Case  of  Linkage. — As  a  typical  example  may  be  taken  tin- 
following  case  of  linkage  in  Drosophila  (Fig.  1  15).  A  male  fly  with  white 
eyes  and  yellow  body  is  mated  to  a  female  with  red  eyes  and  gray  body. 


linkage:      in      drosophila 


<M 


s 


99% 


1% 


L 


Fig.  145.-Linkage  in  DrosophUa.     Red  eyes  in  black;  gray  bodiea  stippled;   *hit 

and  yellow  bodies  unshaded. 

The  flies  of  the  F1  generation  all  have  red  eyes  and  .may  bodies,  since  red 
is  dominant  over  its  allelomorph  while,  and  may  Is  dominanl  over  its 
allelomorph  yellow.  If,rnow,  the  females  of  this  generation  are  back- 
crossed"  to  males  with  both  of  the  recessive  characters,1  white  and  yellow, 
iThis  back-crossing  to  the  pure  recessive  is  the  common  method  of  testing  the 
genotypic  constitution  of  hybrids. 


380  INTRODUCTION  TO  CYTOLOGY 

flies  of  four  types  appear  in  the  next  generation,  as  shown  in  the  dia- 
gram: white-yellow,  white-gray,  red-yellow,  and  red-gray.  Since  one 
parent  in  this  last  cross  is  a  double  recessive,  these  results  show  that 
the  Fi  red-gray  female  must  produce  eggs  of  these  four  genotypic 
constitutions. 

All  four  types,  however,  are  not  produced  in  equal  numbers.  Those 
flies  with  the  same  combinations  as  were  shown  by  their  grandparents, 
white-yellow  and  red-gray,  together  comprise  99  per  cent  of  the  total 
number;  only  1  per  cent  show  the  other  possible  combinations,  white- 
gray  and  red-yellow.  It  thus  appears  that  if  the  two  characters,  red 
eyes  and  gray  body,  enter  a  hybrid  together  (i.e.,  are  contributed  by  the 
same  parent)  they  come  out  together  in  the  next  generation  in  nearly  all 
cases:  they  are  definitely  linked,  and  this  is  explained  on  the  hypothesis 
that  their  genes  are  located  in  the  same  chromosome.  The  same  is 
obviously  true  of  their  respective  allelomorphs,  white  eyes  and  yellow 
body:  their  genes  are  carried  in  the  other  chromosome  of  the  homologous 
pair.  Were  the  two  allelomorphic  pairs  of  genes  carried  in  different  pairs 
of  chromosomes  no  such  linkage  would  occur :  the  two  characters  red  and 
gray,  and  similarly  the  two  characters  white  and  yellow,  would  then  be 
present  together  in  50  per  cent  of  the  flies,  the  chance  frequency,  rather 
than  99  per  cent.  How  it  is  that  the  linkage  is  broken  in  some  cases, 
giving  1  per  cent  with  exceptional  combinations,  is  a  question  to  which 
we  shall  return  later  in  the  chapter. 

Sex-linkage. — A  very  interesting  special  case  of  linkage  is  seen  in 
the  phenomenon  of  sex-linkage,  which  may  be  illustrated  by  the  following- 
example  (Fig.  146)  (Morgan  1910).  If  a  red-eyed  wild  male  is  mated  to  a 
white-eyed  female  (a  member  of  a  race  descended  from  white-eyed 
mutants)  the  F\  individuals  are  white-eyed  males  and  red-eyed  females — 
each  eye  color  has  been  transferred  from  one  sex  to  the  other,  a  case  of 
" criss-cross ';  inheritance.  If  these  Fi  flies  are  bred  together,  the  F2 
generation  comprises  four  types :  red-eyed  males  and  females,  and  white- 
eyed  males  and  females.  Turning  now  to  the  chromosomes,  if  the  dis- 
tribution of  the  sex-chromosomes  is  followed  through  these  generations 
this  striking  fact  is  revealed:  red  eye  color  appears  in  every  fly,  male  or 
female,  which  possesses  the  X-chromosome  of  the  original  red-eyed  male; 
and  white  eyes  appear  in  every  male  which  receives  one  of  the  X-chromo- 
somes  of  the  original  white-eyed  female,  and  in  every  female  receiving- 
two  of  them.  This  is  taken  by  Morgan  to  mean  that  the  original  male's 
X-chromosome  carries  the  dominant  factor  for  red  eyes,  while  each  of 
the  X-chromosomes  of  the  original  white-eyed  female  carries  a  recessive 
factor  for  white  eyes.  In  all  the  flies  it  is  seen  that  the  presence  of  one 
X-chromosome  is  correlated  with  maleness,  and  that  of  two  X-chromo- 
somes with  femaleness  (compare  Fig.  140);  and  that  the  two  types  of 
eye  color  under  consideration  follow  the  distribution  of  these  chromo- 


LINKAGE 


381 


somes — they  are  sex-linked  characters.*  So  far  as  is  known,  tin-  )  - 
chromosome  of  the  male  carries  no  sex-  or  sex-linked  factors.  This 
general  interpretation  is  directly  applicable  to  the  reciprocal  crosa  white- 
eyed  male  X  red-eyed  female),  in  which,  however,  the  relative  proportions 
of  red-eyed  and  white-eyed  flies  in  l<\  and  Fj  arc  different  :  in  i\  all  flies 
of  both  sexes  have  red  eyes,  while  in  F2  all  the  females  and  one-half  of 
the  males  have  red  eyes,  white  eyes  appearing  only  in  one-half  of  the 
males.  (See  Morgan  et  al.  1915,  pp.  10-20;  Babcock  and  ( Jlausen  1918, 
pp.  74-77.) 


EDOlOrnllA 


>  ^0 


Fig.  146. — Sex-linkage  in  DrosophUa.     Three  successive  generations  al   left;  red  • 
shown  in  black.     The  history  of  the  sex-chromosomes  through  these  generations  sho*  D  a1 
right;  X- chromosome  of  original  male  shown  in  black.      (Adapted  from  Morgo 

In  such  cases  as  the  above  it  is  evident  that  characters  other  than 
sex  may  be  referred  to  certain  chromosomes  of  the  complement:  if  is 
possible  not  only  to  tell  which  chromosomes  have  to  do  with  sex,  but 
also  to  identify  the  ones  concerned  in  the  production  of  red  and  white 
eye  colors.  A  large  number  of  such  sex-linked  characters  haw  been 
identified  in  DrosophUa,  and  several  have  been  found  in  other  animals. 
Human  colorblindness  is  a  character  which  is  inherited  in  a  manner 
analogous  to  that  of  sex-linked  characters  in  Drosophila,  and  its  me- 
chanism is  apparently  the  same.  The  presence  of  this  detect  more 
commonly  in  men  than  in  women,  and  its  appearance  in  so  few  individ- 
uals in  affected  lines,  are  due  to  the  tact  that  it  is  both  a  recessive  and  a 

i  Sex-linked  characters  are  not  to  be  confused  with  Bex-limited  characters.     The 
latter  are  those  found  exclusively  in  one  sex,  and  are  now  referred  to  as  secondary 

.sexual  characters. 


382  INTRODUCTION  TO  CYTOLOGY 

sex-linked  character,  precisely  like  white  eyes  in  Drosophila.  It  occurs 
in  a  woman  only  if  both  of  her  X-chromosomes  bear  factors  for  it, 
which  means  that  such  a  factor  must  have  been  received  from  each 
parent ;  whereas  one  such  factor  is  sufficient  to  produce  colorblindness  in 
a  man,  because  his  F-chromosome  carries  no  factors  which  might  domi- 
nate it.  Furthermore,  since  the  X-chromosome  of  the  male  is  always 
derived  from  the  mother,  a  man  can  inherit  colorblindness  from  his 
mother  but  not  from  his  father.  From  these  facts  it  follows  that  a 
colorblind  woman  transmits  the  defect  to  all  of  her  sons  and  to  half  of 
her  grandsons  and  granddaughters;  whereas  a  colorblind  man  transmits 
the  defect  to  none  of  his  children  and  only  to  one-half  of  his  grandsons.1 

The  first  sex-linked  character  known  in  plants  was  that  of  narrow 
rosette  leaves  in  the  red  campion,  Lychnis  dioica  (Shull  1910,  1911),  a 
plant  which  appears  to  have  the  XY  type  of  sex  inheritance,  but  in  which 
no  distinguishable  sex-chromosomes  have  been  identified. 

Non-disjunction. — The  chromosome  interpretation  of  sex  and  of 
sex-linkage  has  received  an  interesting  confirmation  in  a  phenomenon 
discovered  by  Bridges  (1913).  In  a  certain  strain  of  Drosophila  the 
white-eyed  females  were  observed  to  give  rise  to  a  certain  proportion  of 
"  unexpected  "  forms.  Most  of  their  offspring  (92  per  cent)  were  red-eyed 
females  and  white-eyed  males,  as  expected  in  such  an  experiment,  but 
some  of  them  (8  per  cent)  were  white-eyed  females  and  red-eyed  males. 
A  long  series  of  crosses  showed  that  these  results  could  be  accounted  for 
if  it  were  assumed  that  in  the  original  white-eyed  female  both  of  the  X- 
chromosomes  passed  together  to  one  pole  in  the  reduction  division  instead 
of  separating  as  they  should.  This  was  termed  non-disjunction  (Fig.  147). 
As  a  result  the  eggs,  instead  of  having  the  normal  single  X-chromosome, 
would  have  either  two  or  none,  and  the  distribution  of  the  sexes  and  the 
sex-linked  characters  would  be  altered  in  the  manner  observed.  In  a 
cytological  examination  of  the  flies  in  which  these  abnormal  phenomena 
appear  Bridges  showed  that  this  non-disjunction  of  the  X-chromosomes 
does  occur  occasionally  in  the  female  (Fig.  148).  The  chromosome 
theory  thus  received  confirmation.  "An  abnormal  distribution  of  the 
sex-chromosomes  goes  hand  in  hand  with  an  abnormal  distribution  of  all 
sex-linked  factors"  (Morgan).  Additional  genetic  and  cytological  data 
have  since  been  furnished  by  Bridges  (1916)  and  Safir  (1920). 

Linkage  Groups. — An  extensive  series  of  studies  on  linkage  relations 
in  various  plants  and  animals  has  brought  out  the  fact  that  the  Mendel  ian 
characters  of  a  given  organism  fall  into  a  certain  number  of  "linkage 
groups,"  the  members  of  each  group  being  linked  to  one  another  in 
various  degrees  but  showing  no  linkage  with  the  members  of  other  groups. 
It  appears  further,  when  the  relationships  of  enough  characters  have 
been  worked  out,  that  the  number  of  linkage  groups  is  the  same  as  that 

1  This  case  is  fully  explained  by  Babcock  and  Clausen  (1918,  p.  197). 


LI \  KAGE 


of  the  chromosome  pairs.     Drosophila  melanogaster,  in  which  Linkage 
relations  have  been  most  fully  analysed,  has  lour  pairs  of  chromoson 
(Fig.  148):  two  large  "euchromosome"  pairs,  one  pairofsex-chromosom< 


DROSOPHILA 

MOM       Pi  '      UM(   ; 


X 

KIN  J  J 
Of 

'X' 

f 

HINDS 


toos 


/* 


0"~"  ^ 


T  )     (         *tY     )(      ,Xt   j     f       tt    j 


Fig.   147. — Non-disjunction  and  its  results  in  Drosophila.     The  two  large  circles   in 
first  row  represent  male  and  female  flies  producing  sperms  and  eggs  respectively.      Non- 
disjunction in  the  female  gives  2  kinds  of  eggs,  with  XX  and  with  no  Bex-chromoson 
instead  of  the  normal  single  kind  with  one  X.     At  fertilization  there  are  possible  1  combi- 
nations rather  than  2,  as  shown  in  the  large  circles  of  second  row.     I  ►wing  to  th<'  wveral 
ways  in  which  her  3  sex-chromosomes  may  be  distributed  at  maturation,  tin   female  repre- 
sented by  the  third  circle  produces  4  kinds  of  eggs.     When  mated  to  a  normal  male  (below 
horizontal  line)  with  his  2  kinds  of  sperms,  8  combinations  are  possible  (las!  row).      Nos 
1,  4,  and  5  are  normal  flies  and  give  the  usual  types  of  progeny.      Nos.  2,  6,  and  7.  owing 
to  the  presence  of  3  sex-chromosomes,  give  exceptional  results  when  bred.      Types   N 
and  No.  8  do  not  appear  in  the  cultures,  probably  because  they  die  very   early.       1  la- 
original  male  has  red  eyes  and  the  original  female  white  eyes.      Red  eyes  "•'!  by 
dots)  appear  in  every  fly  bearing  the  X-chromosome  of  the  original  male  as  in   1 
Compare  Morgan  1919a'  Figs.  93  and  94.      (Diagram  based  on  <lntn  of  Bridges  mid  M 

A  A  ^| 


A 


? 


J* 


? 


Fig.  148.- 


The  chromosomes  of  Drosophila  >n<  lanogasU  r  :i-  they  appear  during  mitosis  in  a 
female,  a  male,  and  a  non-disjunctional  female.     I  I         Morgan.) 


and  one  pair  of  very  small  " m-chromosomes."  The  Mendelian  charac- 
ters in  Drosophila  fall  into  four  linkage  groups,  and  it  is  noteworthy 
that  one  of  these  groups  contains  only  two  known  characters,     Each 


384  INTRODUCTION  TO  CYTOLOGY 

chromosome  pair  therefore  seems  to  be  responsible  for  a  certain  group 
of  characters.  It  has  been  shown  above  that  one  of  these  groups, 
the  sex-  and  sex-linked  characters,  can  be  definitely  assigned  to  the  pair 
of  sex-chromosomes;  and  Morgan  further  believes  that  the  factors 
for  the  two  characters  of  the  small  linkage  group  are  located  in  the 
m-chromosomes.  The  two  remaining  linkage  groups,  which  contain 
many  characters  each,  are  assigned  to  the  large  euchromosome  pairs. 
Each  chromosome  is  accordingly  regarded  as  a  body  containing  the  fac- 
tors or  genes  for  a  considerable  number  of  characters;  and  on  the  basis 
of  the  evidence  to  be  presented  below  it  is  concluded  that  these  genes, 
differing  thus  in  their  hereditary  potencies,  are  arranged  in  the  chromo- 
some in  a  linear  series  as  suggested  by  Roux  many  years  ago. 

In  plants  the  best  known  cases  of  linkage  are  in  Zea  Mays,  in  which 
Emerson  and  his  students  at  Cornell  have  identified  six  linkage  groups, 
and  Pisum,  which  has  so  far  shown  four  linkage  groups  (White  1917). 
Since  maize  has  10  pairs  of  chromosomes,  four  more  groups  may  be 
expected,  while  in  Pisum,  which  has  seven  pairs,  three  more  groups  will 
probably  be  established;  in  fact  seven  independently  inherited  characters 
are  known.  It  is  an  interesting  fact  that  Mendel,  in  his  famous  researches 
on  Pisum,  happened  to  select  for  study  seven  pairs  of  characters  belonging 
to  the  seven  different  groups,  and  so  did  not  detect  the  phenomenon  of 
linkage. 

From  the  foregoing  considerations  there  arises  an  interesting  and 
very  important  question.  If  two  homologous  chromosomes,  each  carry- 
ing factors  for  a  certain  group  of  characters  (those  of  one  group  allelo- 
morphic  to  those  of  the  other  group),  separate  into  different  gametes 
(or  spores)  at  the  time  of  reduction,  how  does  it  happen  that  occasionally 
there  appears  an  individual  with  some  of  the  characters  of  each  group? 
And  if  a  single  chromosome  carries  a  series  of  factors  for  a  certain  group 
of  characters,  how  shall  we  account  for  the  occasional  individual  with 
some  of  these  characters  but  not  the  rest?  To  state  the  problem  in  the 
terms  of  linkage,  if  each  group  of  linked  characters  is  represented  by  a 
series  of  genes  in  a  given  chromosome,  how  is  the  linkage  broken  in 
a  certain  percentage  of  cases,  with  the  resulting  formation  of  new  link- 
age groups,  as  shown  by  the  exceptional  red-yellow  and  white-gray  flies 
in  the  experiment  described  at  page  379?  A  solution  to  this  problem  has 
been  offered  in  the  Chiasmatype  Theory. 

The  Chiasmatype  Theory. — In  our  discussion  of  chromosome  con- 
jugation it.  was  pointed  out  (p.  257)  that  various  opinions  have  been 
entertained  regarding  the  nature  of  the  association  between  the  members 
of  the  synaptic  pair.  Some  workers  have  held  that  the  chromosomes 
fuse  completely  and  lose  their  identity,  and  that  the  two  chromosomes 
appearing  on  the  first  maturation  spindle  are  not  to  be  looked  upon  as 
identical  with  those  which  entered  into  conjugation.     On  the  contrary, 


LINKAGE 


385 


there  are  those  who  deny  any  fusion  a1  all  between  the  members  of  the 
pair,  holding  rather  that  their  identity  is  do1  impaired  in  any  way  during 


Fig.  149. — The  behavior  of  the  conjugating  homologous  chromosomes  according  t-> 
the  Chiasmatype  Theory  of  Janssens.     Single  crossing  over  at  left;  double  crossing  over 

at  right.      (Adapted  from  Babcock  and  Clausen.) 


CHIA3I1ATYPC  •  JAN33EM5       I90». 


0 


v> 


0 


00 


Fig.   150. — Crossing  over  between  2  of  the  4  chromatids  of  the  chromosome  tetrad,  giving 
2  crossover  and  2  non-crossover  gametes.      (Adapted  from  Janssens,  1909 


Fig.  151. —  Chromosomes  of  Balracose.ps  attenuatns,  Bhowing  chiasmas.     (After  Janssens.) 

their  intimate  association:  the  two  chromosomes  appearing  on  the  first 
maturation  spindle  are  exactly  the   same  as   those   which   conjugated. 


386  INTRODUCTION  TO  CYTOLOGY 

Between  these  two  extremes  lie  other  views,  the  most  suggestive  of  them 
being  that  proposed  by  Janssens  (1909).1 

The  theory  of  Janssens  in  its  simplest  possible  form  may  be  stated  as 
follows.  The  members  of  the  conjugating  pair  twist  about  each  other 
and  come  into  very  intimate  association  at  certain  points.  When  they 
again  separate  a  break  occurs  at  the  point  or  points  of  closest  contact, 
but  along  a  new  plane,  so  that  each  of  the  two  separating  chromosomes 
is  made  up  of  portions  of  both  conjugating  members  (Fig.  149).  This 
process  is  known  as  chiasmatypy  or  crossing  over.  Such  a  behavior  might 
occur  at  various  stages  in  the  heterotypic  prophase:  most  probably  it 
takes  place  at  an  early  stage,  when  the  conjugating  chromosomes  are  in 
the  form  of  simple  thin  threads  (Fig.  83).  In  other  cases  it  may  take 
place  at  a  later  stage,  when,  in  the  case  of  animals,  each  of  the  chromo- 
somes has  split  preparatory  to  the  second  mitosis,  forming  a  tetrad  of 
chromatids.  Here  the  crossing  over  may  occur  between  only  two  of  the 
four  chromatids  (Fig.  150,  Janssens's  typical  case;  see  also  Fig.  151), 
or  between  all  four.  If  only  two  of  the  four  chromatids  are  concerned, 
only  two  of  the  resulting  gametes  (or  spores)  will  be  " crossover  gametes" 
(or  spores),  as  in  Fig.  150;  whereas,  if  the  crossing  over  takes  place  between 
all  four  of  the  chromatids,  or  between  the  two  yet  unsplit  threads  in  the 
earlier  prophase,  all  four  of  the  gametes  (or  spores)  will  be  "  crossover 
gametes"  (or  spores). 

Application  of  the  Chiasmatype  Theory  to  the  Problems  of  Linkage. — 
It  is  the  above  interpretation  of  the  nature  of  chromosome  conjugation 
that  lies  at  the  basis  of  the  work  of  Morgan  and  his  students  on  Drosophila. 
As  already  pointed  out,  these  workers  have  found  good  evidence  for  the 
conclusion  that  each  chromosome  is  responsible  for  a  certain  group  of 
characters,  the  members  of  the  group  showing  a  strong  tendency  to  remain 
associated  because  their  genes  are  borne  by  a  common  carrier.  They 
further  believe  that  the  evolution  of  new  character  groupings  has  been 
brought  about  not  only  through  the  crossing  of  different  hereditary 
strains,  but  also  through  the  evolution  of  chromosomes  with  new  con- 
stitutions by  the  process  of  crossing  over.  On  the  basis  of  the  frequencies 
in  which  the  new  types  of  grouping  occur  the  relative  positions  (loci) 
of  the  genes  for  the  different  characters  have  been  plotted  in  the 
chromosomes. 

The  above  points  are  illustrated  in  Fig.  152,  which  summarizes  what 
is  supposed  to  have  occurred  in  a  certain  series  of  crosses  between  flies 
with  yellow  body,  white  eyes,  and  miniature  wings,  and  flies  with  gray 
body,  white  eyes,  and  long  wings.     In  the  oells  of  the  hybrid  there  is 

1  Janssens  has  recently  (1919«l>)  published  an  outline  of  his  views  of  the  maturation 
phenomena  in  Orthoptera  in  which  he  again  makes  use  of  the  chiasmatype  interpreta- 
tion. His  results  are  discussed  in  some  detail  from  the  cytological  and  genetic  points 
of  view  by,, Wilson  and  Morgan  (1920), 


LINKAGE 


387 


a  chromosome  from  one  parenl   bearing  the  genes  for  the  three  linked 

characters,  Y,  W,  and  M,  and  a  chromosome  from  the  other  parenl  with 
the  genes  for  the  three  linked  characters,  G}  /.'.  and  L.  The  first  three 
characters  are  allelomorphic  to  the  last  three  respectively,  and  the  two 
chromosomes  are  homologous.1  [JpoD  breeding  from  the  females  of 
these  hybrids  it  is  found  that  in  the  majority  of  cases  (firsl  column)  the 
F2  flies  show  the  same  genetic  consitution  as  do  the  grandparents  with 
respect  to  these  particular  characters).  This  is  taken  to  mean  that  the 
two  homologous  chromosomes  forming  the  synaptic  pair  and  separating 
at  reduction  have  maintained  their  substance   intact- -there   has   been 


DROSOPHILA 


YELLOW  »0»Y 
WHITt  CVCt 
MINIATum     WIN&S 


&WAY      »0»Y 
KID       VtU 

LON»     WINfrj 


^ 


o 


^ 


V      K? 


V 


1361       20S9. 


^ 


^ 


^ 


817       887 


23  17 


O 


Fig.   152. — Diagram  illustrating  the  evolution  of  new  linkage  groups  through  crossing  over. 

Explanation  in  text.     (Adapted  from  Morgan.) 


no  crossing  over.  In  a  certain  number  of  cases  (second  column)  Dew 
groupings  of  the  characters  in  question  are  observed  in  the  Pi  flies:  some 
have  Y,  W,  and  L,  while  others  show  G,  R,  and  M.  This  is  interpreted 
on  the  assumption  that  a  break  has  occurred  along  a  new  plane  (dotted 
line)  at  a  point  of  contact,  so  that  at  reduction  some  of  the  garnet 
and  hence  the  F2  flies  to  which  they  give  rise,  receive  a  chromosome  with 
Y,  W,  and  L,  while  others  receive  G}  /?,  and  M .  In  a  smaller  Dumber 
of  cases  (third  column)  the  new  combination-  YRL  and  QWM  arc  formed 
in  a  similar  manner,  the  break  and  reunion  occurring  ;ti  another  point. 
In  a  very  small  number  of  cases  (fourth  column)  the  combinations 
YRM  and  GWL  appear,  which  can  be  explained  on  1  he  basis  of  a  double 

1  The  hybrids  have  gray  bodies,  red  eyes,  and  long  win^s,  because  a,  B,  and 

L  are  dominant  over  )',   W,  and  M.     (Ordinarily  t he  genes  have  other  designations.) 


CHROMOSOME  I 

0.0  \ 

«4 

/yellow,  scute  0.0  + 
A  lethal  7 
//,  broad 
l/A  prune 

1.5 — 

—  white,  etc. 

3.0-—: 

- — notch,  facet 
abnormal 

5.5 

echinus 

7.3 ; 

7.5-" 

bifid 

"     -  ruby 

13.7 
\6.7 


20.0 
21.0 


27.5 


33.0 

36.0- 
36\1- 

37.5 
38.0 


43.0 
44.4 


53.5 
5^.5 


56 

57, 

58, 
59 


65.0 


croesTeinlese 

club 

cut 
singed 

tan 


Term! lion 

•tiny-bristles 
-miniature 

dusky 
furrowed 


sable 
garnet 


small-wing 
rudimentary 

forked 
bar 

small-eye 
fused 


cleft 


CHROMOSOME  IT 


2.0  telegraph 


0.0 

1.0 


^.0  expanded 


9.0 

11.0 

13.0 

14.0 


85.0 

88.0 


100.0 
101.0 

103.0 
104.5 
105.0 
105.5 


22.3 


28.0 
29.0 


33.0 
35.5 


44.0 

46.5 
46.7 

48.5 
52.5 

58.0 
61.0 


65.0 

66.5 

67.0 

70.0 
71.0 

73.5 
75.0 

77.0 


95.0 

97.5 
98.5 


star 
aristaless 


truncate 

gull 

pink-wing 
streak 


cream-b 


flipper 
dachs 


ski 
squat 


minute-6< 

black 

jaunty 

apterous 


purple 

eafranin 
trefoil 

Testigial 

telescope 
dash 

lobe 
minute-5II 

curved 
dacheoug 

roof 


tninute-2 
humpty 

purpleoid 

aro 
plexu6 

lethal-IIa 
brown 

blistered 

morula 

speck 

Mkloon 


CHROMOSOME  III 


0.0  roughoid 


CHftOMOSOVE  IV 


25.3 
25.8 


32.0     I- 

33.5 
34.0 
35.0 


38.3 
38.5 

40.5 
41.5 

43.0 

!*4.0 
•+5.0 
45.5 


53.8 
54.0 
55.0 
56.0 


59.0 
60.0 
60.5 
61.0 

63.5 

65.5 

67.5 
68.0 

70.0 
72.0 


86.5 
89.0 


95.7 


0.0 


bent 
eyeless  0.0  + 


sepia 
hairy 


divergent 

cream-III 
dwarfoid 

scarlet 


tilt 
dichaete 

ascute 

deformed 

maroon 

curled 

dwarf 

pink 


tv/o-bristle 
spineless 
bi thorax 
tithoraxoid 

glass 
kidney 

giant 
spread 

delta 

hairless 

ebony 
band 

CIII 
white-ocelli 


rough 
beaded 


tlaret 

raim»t* 


hio.  lo3. — The  chromosome  map  of  Drosophila  melanogaster,  showing  the  loci  of  the 

genes  as  determined  by  Morgan  and  his  associates.      Correcte'd  to  November.  1920.      Figure 
kindly  furnished  by  Professor  Morgan. 


LINKAGE 

crossing  over  us  shown  in  the  diagram.  Mosi  of  the  Ft  flies  show  the 
same  combinations  as  their  grandparents,  which  indicates  thai  do  crossing 
over  has  occurred  in  the  majority  of  the  gamete-forming  cells.  Since 
there  are  many  more  F2  individuals  with  YWL  or  GRM  second  column 
than  with  YRL  or  GWM  (third  column),  ii  is  believed  thai  the  distance 
between  the  genes  W  and  M  (and  between  Rand  /.)  in  the  original  chromo- 
some must  be  greater  than  that  between  Fand  W  (and  between  G  and  R 
so  that  there  is  more  chance  for  crossing  over  to  occur  in  the  lower  pan 
than  in  the  upper.  Thus  if  two  linked  characters  arc  often  separated 
(i.e.,  have  their  linkage  broken)  their  genes  are  thought  to  lie  relatively 
far  apart  in  the  chromosome,  whereas  linked  characters  separated  only 
rarely  are  supposed  to  have  their  genes  located  very  near  each  other. 
Recombinations  involving  a  double  or  even  more  complex  crossing  over 
(fourth  column)  would  be  expected  very  rarely.  In  this  way  the  gen 
for  the  various  characters  have  been  assigned  to  their  loci  in  the  chromo- 
somes on  the  basis  of  the  frequencies  in  which  the  various  new  combi- 
nations in  F2  appear. 

In  Fig.  153  is  shown  the  "map™  of  the  chromosomes  of  Drosophila 
as  determined  by  Morgan  and  his  associates,  each  factor  being  placed 
a  certain  number  of  units  of  distance  from  the  end.  Many  other  known 
genes  are  not  shown  in  the  diagram.  As  the  unit  of  distance  is  la  ken 
that  space  separating  two  factors  whose  linkage  is  broken  (i.e.,  between 
which  crossing  over  occurs)  in  1  per  cent  of  the  cases.  Thus  crossing 
over  occurs  between  "yellow"  and  "bifid"  in  7.3  per  cent  of  the  observed 
cases;  the  factors  are  therefore  placed  7.3  units  apart  (first  chromosome 
Furthermore,  if  some  linkage  relations  are  known,  it  is  possible  to  calcu- 
late certain  other  linkages  in  advance.  For  example,  if  it  were  known 
that  crossing  over  occurred  between  "sepia"  and  "pink*'  (third  chromo- 
some) in  20.2  per  cent  of  the  flies,  and  also  that  "sepia"  and  "kidney' 
showed  a  34.7  per  cent  crossing  over,  the  prediction  that  "pink'  and 
"kidney'  would  show  a  14.5  per  cent  crossing  over  (not  including  the 
modifying  effects  of  double  crossing  over,  should  this  occur)  would  In- 
borne  out  by  experimental  results.  Such  an  agreement  of  the  results  of 
new  crosses  with  predictions  made  on  the  basis  of  known  linkages  has 
occurred  over  and  over  again  in  the  experiments  of  Morgan  and  his 
students.     The  chiasmatype  hypothesis  as  thus  elaborated  obviously 

fits  the  observed  facts  remarkably  well. 

Interference. — Another  piece  of  evidence  broughl  forward  in  suppoii  of 
the  hypothesis  that  the  factors  have  a  linear  arrangemenl  within  the 
chromosome  is  the  phenomenon  of  interference,  which  has  been  elucidated 
bySturtevant  (1913),  Weinstein  (1918),  and  particularly  by  Muller.  It 
the  factors  or  genes  are  arranged  in  a  series  as  supposed  on  the  above 
hypothesis,  it  would  be  expected  t  hat  when  crossing  over  occurs  at  a  given 
point  in  a  pair  of  chromosomes,  the  regions  immediately  on  either  side  of 


390 


INTRODUCTION  TO  CYTOLOGY 


d 


f 


if 


this  point  would  be  prevented  from  crossing  over  at  the  same  time,  for  the 
reason  that  the  twisting  of  the  chromosomes  about  each  other  is  not  close 
enough  to  allow  two  crossovers  so  near  each  other.  Thus  in  Fig.  154,  if 
crossing  over  occurred  between  the  two  factor  pairs  Dd  and  Ee,  breaking 
the  linkages  between  D  and  E  and  between  d  and  e,  there  would  be  at  the 
same  time  no  such  break  between  Cc  and  Dd  or  between  Ee  and  Ff, 
since  for  mechanical  reasons  a  second  crossing  over  could  not  take  place 
at  either  of  these  points  simultaneously  with  that  between  Dd  and  Ee. 
Crossing  over  at  one  point  would  thus  interfere  with  crossing  over  which 

might  otherwise  occur  at  nearby  points.  The  amount  of 
this  interference  would  progressively  decrease  at  points 
farther  and  farther  from  the  first  crossover  point,  until 
at  a  certain  distance  (measured  by  the  length  of  the  loops 
usually  formed  by  the  twisting  chromosomes),  as  a,tLM, 
it  would  vanish  entirely;  here  crossing  over  would  occur 
with  its  normal  frequency  irrespective  of  any  crossover 
at  DE. 
*  Muller  has  found  that  the  characters  behave  accord- 

H    ing  to  these  expectations.     If  two  characters  have  their 
linkage   broken   in    a   certain   percentage  of  cases,  this 
.      percentage  is    noticeably  lowered   if  breaks   in  linkage 
occur  between  two  other  characters  having  normally  a 
fairly  close  linkage  with  the  first  two.     In  other  words, 
one  linkage  break  interferes  with  other  linkage  breaks 
within  the  same  linkage  group ;  and  the  degree  of  this 
interference  varies  from  a  high  value  in  the  case  of  a 
closely  linked  series  of  characters  to  zero  in  the  case  of 
characters  very  loosely  linked.     This,  it  is  pointed  out, 
P      is  just  what  should  occur  among  characters  represented 
by  a  linear  series  of  genes  in  chromosomes   which  un- 
dergo crossing  over,  but  which  cannot  twist  about  one 
another  with  more  than  a  certain  degree  of  closeness. 
The  phenomenon  of  interference  thus  indicates  another 
point  in  which  the  chiasmatype  hypothesis  as  developed  by  Morgan  fits 
the  experimental  facts. 

A  further  point  is  of  interest  in  this  connection.  In  Drosophila  it  is 
only  in  the  females  that  crossing  over  takes  place;  it  is  in  the  eggs,  and 
not  in  the  spermatozoa,  that  new  factor  combinations  appear  as  the  result 
of  this  process.  The  absence  of  crossing  over  in  the  male  may  be  asso- 
ciated with  the  fact  that  the  F-chromosome  carries  no  known  factors;1 
the  male  is  heterozygous  for  sex.  In  the  fowl,  in  which  the  female 
rather  than  the  male  is  heterozygous  for  sex,  it  has  been  shown  that 
crossing  over  occurs  in  the  male  but  not  in  the  female.     Crossing  over  for 

^ee,  however,  Castle  (1921). 


V 
M 


K 


>*\ 


H 


Fig.  154.— Dia- 
gram illustrating 
interference.  Ex- 
planation in  text. 


LINKAGE  393 

some  reason  is  limited  in  these  cases  and  some  others  to  the  Bex  which  is 
heterozygous  Tor  the  sex-factors.  <  >n  the  contrary,  in  the  grasshopper, 
Apotettix,  Nabours  (MM!))  has  shown  thai  some  crossing  over  occurs  in 
both  sexes,  and  the  same  appears  to  be  1  rue  in  Primula  (( Iregorj  ;  AJten- 
burg  1916),  the  rat  (Castle  and  Wright  1916),  and  Zea  (Emerson  .  In 
Paratettix,  in  which  no  crossing  over  lias  been  demonstrated,  Miss 
Harman  (1920)  reports  that  the  homologous  chromosomes  do  do1 
conjugate  until  the  end  of  the  prophase,  and  suggests  thai  their  indepen- 
dence during  the  early  stages  may  account  for  the  absence  of  crossing 
over. 

General  Discussion. — In  the  foregoing  pages  a  brief  account  has  been 
given  of  the  main  points  in  the  theory  developed  by  those  who  have  mad. 
the  most  thoroughgoing  attempt  to  relate  the  phenomena  of  heredity  bo  a 
visible  cell  mechanism.  To  follow  out  the  details  of  its  applied  ion  does 
not  lie  within  the  scope  of  this  chapter:  it  is  here  intended  only  to  furnish 
a  starting  point  for  cytological  studies  in  this  field  by  indicating  the 
common  ground  upon  which  cytology  and  genetics  meet.  It  is  import- 
ant, however,  to  differentiate  between  evidence  which  is  genetical  and 
that  which  is  cytological  in  nature;  and  further  to  remind  ourselves  to 
what  extent  observed  fact  and  hypothesis  respectively  have  been  woven 
into  the  theory.  Caution  is  particularly  necessary  in  this  latter  regard. 
since  the  general  nature  of  many  of  our  ideas  of  inheritance  is  traceable 
in  part  to  the  speculative  theories  of  Weismann.  Weismann's  theories 
of  heredity  and  development,  which  are  summarized  in  the  next  chapter. 
were  primarily  "corpuscular"  or  "particulate"  theories:  the  phenomena 
of  heredity  and  development  were  referred  to  distinct  material  units 
which  in  some  way  were  able  to  bring  about  the  development  of  I  In- 
heritable characters  in  the  individual  and  their  transmission  from  one 
generation  to  the  next.  Bearing  in  mind  the  phenomena  of  inheritance 
reviewed  in  the  preceding  chapters,  especially  the  behavior  of  the  Men- 
delian  characters,  it  is  difficult  to  escape  the  conclusion  that  differential 
factors  of  some  sort,  which  in  an  unknown  manner  initiate  the  series  of 
reactions  resulting  in  the  several  characters,  are  carried  in  the  nucleus. 
To  determine  the  nature  of  these  factors  and  to  discover  the  real  relation 
existing  between  them  and  the  developed  characters  are  among  our 
greatest  problems.  That  the  factors  or  genes  are  discrete  units  is  a 
hypothesis  which  is  not  only  plausible,  but  has  also  proved  itself  to  be 
most  useful.  If  such  factors  exist,  the  chromosomes  afford  a  means  of 
precisely  the  kind  required  to  account  for  the  observed  distribution  of 
characters  throughout  a  series  of  generations.  Hence  from  Roux  and 
Weismann  onward  the  factors  have  been  lodged  in  the  chromosomes. 

But  it  is  when  these  factors  are  directly  sought  with  the  aid  of  the 
microscope  that  disappointment  is  met.  The  frequently  observed 
granules  or  chromomeres  in  the  chromatin  thread  or  chromosome  are 


392  INTRODUCTION  TO  CYTOLOGY 

accepted  by  some  geneticists  as  the  desired  material  genes;  but,  as  pointed 
out  in  the  chapters  dealing  with  somatic  mitosis  and  reduction,  many 
cytologists  are  very  uncertain  as  to  the  morphological  status  and  signifi- 
cance of  these  bodies,  which  seem  to  them  to  be  far  too  inconstant  in 
number  and  behavior  to  represent  the  units  in  question.  Although  it  is 
tempting  to  look  upon  the  chromatic  granules  as  the  units  which  current 
theories  of  heredity  seem  to  require,  it  must  be  admitted  that  the 
observational  evidence  is  insufficient  to  warrant  the  categorical  state- 
ments frequently  made  to  the  effect  that  the  chromosome  is  composed  of 
a  definite  number  of  more  elementary  visible  chromatic  units,  which 
have  definite  space  relations  and  are  the  significant  units  in  the  cellular 
mechanism  of  heredity.  On  the  other  hand,  the  careful  observations  of 
Wenrieh  (1916)  have  shown  that  in  the  grasshopper,  Phrynotettix  (Fig. 
155),    the  chromatic  granules  are  relatively  constant  in  size  and  position 


c        *        e  f        g  h  i  & -^ 

fc 

Fig.  155. — Chromosome  pair  "B"  in  conjugation  from  the  spermatocytes  of  13  differ- 
ent individuals  of  Phrynotettix  magnus,  showing  constancy  in  size  and  arrangement  of  the 
principal  chromomeres.  The  same  constancy  is  shown  in  the  different  cells  of  a  single 
individual.      X  1500.      {After  Wenrieh,  1916.) 

in  a  given  member  of  the  chromosome  complement,  even  in  different 
individuals;  and  they  furthermore  show  a  close  correspondence  in  the  two 
homologous  chromosomes  as  they  pair  at  synapsis.  This  is  one  of  the 
most  striking  pieces  of  direct  cytological  evidence  yet  brought  forward  in 
support  of  the  theory  that  the  chromosome  is  a  "chain  of  factorial  beads '; 
(Harper),  and  heightens  the  probability  that  the  postulated  units  of 
inheritance  will  turn  out  to  be  more  than  purely  conceptual  ones. 

Whatever  may  be  the  value  of  the  chromatic  granules,  one  can  hardly 
fail  to  recognize  the  highly  suggestive  nature  of  the  arrangement  of  the 
chromatin  in  a  thin  thread,  its  frequently  beaded  appearance,  and  its 
accurate  longitudinal  fission  into  two  equal  parts  at  the  time  of  cell- 
division.  In  the  absence  of  direct  and  convincing  cytological  evidence 
for  the  presence  of  various  "qualities'3  arranged  in  a  series  along  the 
thread,  we  may  still  look  hopefully  for  the  support  which  it  would  seem 
that  the  theory  of  Roux  must  sooner  or  later  have.  It  must  be  admitted 
that  at  present  the  evidence  for  the  existence  of  genes  is  in  the  main 
genetical  rather  than  cytological. 


LINKAGE 


Similarly  unsatisfactory  is  the  cytological  evidence  for  the  breaking 
and  reunion  of  the  chromal  in  1  hreads  required  by  1  !)<■  crossing  over  hypo- 
thesis. Since  the  hypothesis  was  pu1  forward  byJanssens  L909)  adequate 
and  convincing  descriptions  of  this  process  have  been  singularly  wantii 


A 


B 


KJ 


A1 


I 
I 

I 


«•  ft 

v      B 

e      C 


t       I 


B1 


r\ 


I 

I 

I 


B 


I 


Fig.  156. —  Diagrams  illustrating  various  possibilities  concerning  tin-  compound  ring 
tetrads  in  Orthopteran  spermatocytes,  following  the  outlines  <>f  Janssens'a  figures,  bu1 
showing  also  the  relations  of  the  chromatids.  At  the  left  in  each  of  the  upper  figures  i-  the 
longitudinal  tetrad-rod  from  which  the  riim-series  arises,  showing  results  of  assumed  early 
cross-overs  in  Bl  and  C1.  A,  the  compound  ring  as  conceived  by  Mc<  Hung.  Robertson,  etc., 
with  the  four  resulting  chromatids  at  .1'  (no  cross-overs) .  H,  a  compound  ring,  Buch  as 
might  follow  a  two-strand  cross-over  at  each  code,  giving  the  results  shown  in  IV.  I 
compound  ring  giving  the  results  shown  in  Janssens'a  diagrams,  resulting  from  a  two- 
strand  cross-over  between  two  pairs  of  threads,  in  regular  alternation  at  successive  nodes. 
The  result  (i'x)  is  four  classes  of  chromatids,  as  shown  in  Cl.  (Ft<rttr<  ami  legend  from 
Wilson  and  Morgan,  1020.) 

particularly  in  those1  cases  in  which  experimental  results  would  make  its 
establishment  most  desirable.  Wilson  (19126),  Robertson  (1916),  and 
Wenrich  (1916,  1917)  point  out  thai  the  figures  formed  by  the  chromo- 
some  tetrads  in  the  spermatogenesis  of  certain  insects  may  be  interpreted 


394  INTRODUCTION  TO  CYTOLOGY 

without  recourse  to  the  hypothesis  of  chiasmatypy,  and  that  the  observa- 
tions and  figures  of  Janssens  do  not  prove  the  existence  of  that  phe- 
nomenon. Robertson,  for  example,  holds  that  the  "chiasma"  figures  are 
more  simply  explained  as  the  result  of  a  tendency  on  the  part  of  the  four 
chromatids  to  open  out  partly  along  the  conjugation  plane  and  partly 
along  the  plane  of  splitting,  without  any  actual  breaking  and  recombina- 
tion (Fig.  156).  Wilson  (Wilson  and  Morgan  1920),  however,  thinks  it 
"highly  probable  that  the  cytological  mechanism  of  crossing-over  must  be 
sought  in  some  process  of  torsion  and  recombination  in  the  earlier  stages 
of  meiosis — perhaps  during  the  synaptic  phase  or  slightly  later — and  that 
this  process  may  leave  no  visible  trace  in  the  resulting  spireme-threads." 

During  the  time  when  the  homologous  chromosomes  in  the  form  of 
slender  threads  are  twisted  about  each  other  in  the  early  prophase  of  the 
heterotypic  mitosis  there  is  abundant  opportunity  for  the  required  break- 
ing and  union  to  occur,  and  appearances  often  lend  themselves  well  to 
such  an  interpretation.  If  the  side-by-side  position  is  not  assumed  until 
later  in  the  prophase  (Scheme  B,  Chapter  XI)  the  time  during  which  such 
interchanges  might  occur  is  much  shorter.  But  it  is  a  matter  of  extreme 
practical  difficulty  in  either  case  to  determine  whether  or  not  the  plane  of 
separation  is  the  same  as  the  plane  of  union  at  a  given  crossing  point. 
The  forces  controlling  the  breaks  and  recombinations  as  well  as  the  fre- 
quencies with  which  they  occur  in  the  different  chromosome  pairs  are 
even  more  difficult  to  imagine.  The  difficulty  of  accounting  for  these 
phenomena,  however,  does  not  weigh  heavily  as  an  argument  against 
their  occurrence.  Whether  or  not  chiasmatypy  actually  takes  place  is  a 
question  which  must  be  settled  primarily  by  direct  evidence,  and  the  need 
for  careful  search  for  such  evidence  cannot  be  too  strongly  emphasized.  In 
the  opinion  of  the  cytologist  the  behavior  of  each  chromosome  as  a  whole 
must  be  much  more  thoroughly  known  before  cytological  interpretations 
of  the  phenomena  of  inheritance  involving  any  smaller  units  which  the 
chromosome  may  contain  can  be  regarded  as  more  than  hypotheses, 
valuable  as  these  hypotheses  may  be  in  the  correlation  of  genetic  data. 

At  this  point  it  may  be  well  to  recall  (see  Chapter  XI)  that  all  cytolo- 
gists  do  not  agree  that  the  synaptic  mates  maintain  such  an  independ- 
ence (except  at  crossover  points)  as  is  presupposed  by  the  advocates  of 
the  chiasmatype  theory.  A  number  of  observers  have  reported  an  actual 
fusion  of  the  conjugating  threads,  the  resulting  pachytene  thread  sub- 
sequently splitting,  probably  along  the  line  of  this  fusion.  It  may 
accordingly  be  suggested,  in  line  with  the  hypotheses  discussed  by  Allen 
(1905),  that  if  the  threads  actually  carry  or  consist  of  discrete  units  or 
genes,  and  if  these  units  do  not  themselves  fuse  during  the  process,  such 
a  resplitting  of  the  pachytene  thread,  if  not  wholly  along  the  fusion  plane 
because  of  a  twist  of  the  thread  or  of  the  plane  itself,  would  bring 
about  a  redistribution  of  the  genes  as  effectively  as  would  the  chiasmatype 


LINKAGE  :\\\; 


•  • 


process  as  originally  proposed.1  The  splitting  of  chromatic  threads  is, 
moreover,  a  process  already  known  to  occur  in  all  other  mitoa  Bui 
interpretations  involving  the  actual  fusion  of  the  conjugating  threads 
have  been  adversely  criticized  with  much  effecl  by  Gr^goire  1 1910),  and 
only  further  research  on  the  cell  can  lead  us  to  an  adequate  evaluation  of 
the  above  outlined  suggestion. 

Other  Theories  of  Linkage.- -The  chiasmatype  hypothesis  i-  doI  the 
only  one  which  has  been  advanced  to  account  for  the  phenomenon  of 
linkage.  Most  prominent  among  other  attempts  to  solve  this  problem  is 
that  of  the  English  geneticists,  especially  Bateson,  Punnett,  and  Trow, 
who  have  advanced  what  is  known  as  the  Reduplication  Hypothesis.  In- 
stead of  accounting  for  the  new  factor  combinations  manifested  by  a  cer- 
tain percentage  of  the  gametes  on  the  basis  of  an  interchange  of  factors  in 
the  chromosomes  at  the  time  of  reduction,  these  investigators  seek  to 
explain  them  by  postulating  a  series  of  differential  divisions  in  the  earlier 
cells  of  the  germ  cell  lineage,  whereby  Mendelian  factors,  no1  carried  by 
chromosomes,  are  segregated  in  such  a  manner  that  the  observed  types  of 
gametes  are  produced.  Furthermore,  the  various  factors  are  supposed  to 
be  segregated  successively,  and  at  such  stages  in  the  cell  lineage  thai  the 
proliferation  or  reduplication  of  the  cells  with  new  combinations  of  factors 
shall  account  for  the  ratios  in  which  the  new  types  appear.  Although 
the  differentiation  of  the  somatic  and  early  germ  cells  is  accompanied  In- 
visible differences  in  the  constitution  of  their  cytoplasm  (see  p.  406  , 
there  is  at  hand  no  cytological  evidence  for  such  a  segregation  of  heredi- 
tary units  as  is  thought  to  occur  by  the  proponents  of  the  Reduplication 
Hypothesis.  So  long  as  this  is  the  case,  discussion  of  the  hypothesis, 
together  with  the  subhypotheses  formulated  to  meet  certain  serious  objec- 
tions, hardly  belongs  to  cytology.  One  fact,  however,  pointed  out  by 
Morgan  (1919a)  as  very  significant  in  this  connection,  is  found  in  the 
results  of  some  experiments  by  Plough  (1917).  Plough  investigated  the 
effects  of  temperature  on  the  frequency  of  linkage  breaking  (crossing  over 
in  Drosophila.  Not  only  did  he  find  that  temperature  does  affect  the 
amount  of  crossing  over,  but  the  effect  was  clearly  produced  at  the  time 
of  maturation  and  not  earlier.  This  evidence  is  directly  opposed  to  the 
view  that  the  new  factor  combinations  are  formed  during  cell-divisions 
some  time  prior  to  reduction. 

Another  effort  to  account  for  the  results  of  crossing  over  without 
resorting  to  the  chiasmatype  process  is  represented  in  a  suggestion 
(Goldschmidt  19176)  to  the  effect  thai  the  two  factors  of  an  allelomorphic 
pair  are  held  to  their  places  in  the  two  homologous  chromosomes  by  a 
pair  of  variable  forces,  which  allow  them  to  exchange  places  in  a  certain 

lA  form  of  this  interpretation  of  synapsis  has  suggested  itself  also  to  Dr. 
C.  W.  Metz  and  Dr.  E.  G.  Anderson,  who  inform  the  author  that  such  a  hypothesis 
will  in  all  probability  conform  satisfactorily  to  the  data  of  genetics. 


396  INTRODUCTION  TO  CYTOLOGY 

proportion  of  cases  without  involving  a  rupture  of  the  chromosomes.  These 
forces,  however,  are  purely  conjectural.  It  is  pointed  out  by  Jennings 
(1918),  moreover,  that  the  gametic  ratios  theoretically  resulting  from 
such  a  process  do  not  agree  with  the  actual  ratios  observed  in  Drosophila. 

Value  of  the  Chromosome  Theory  of  Heredity. — Whatever  judgment 
may  ultimately  be  rendered  on  the  chromosome  theory  of  heredity  as 
outlined  in  these  chapters,  it  must  be  agreed  that  the  value  of  this  theory 
in  the  present  state  of  our  knowledge  can  hardly  be  overestimated. 
Through  its  use  a  huge  number  of  the  observed  facts  of  inheritance  are 
being  reduced  to  order:  the  painstaking  investigation  of  the  interrelation- 
ships of  all  the  known  heritable  characters  of  even  a  single  organism 
such  as  Drosophila  cannot  fail  to  be  a  great  service  to  biological  science. 
Its  appeal  to  the  cytologist,  as  Wilson  states,  is  largely  through  the  man- 
ner in  which  it  seeks  to  make  use  of  known  cell  mechanisms  rather  than 
entirely  h3rpothetical  processes.  Those  portions  of  the  theory  which 
are  as  yet  unsupported  by  the  results  of  direct  cytological  observation, 
though  not  contradicted  thereby,  at  least  have  the  virtue  of  affording  a 
useful  and  graphic  representation  of  the  mutual  behavior  of  hereditary 
characters.  Notwithstanding  the  statement  that  "  the  graphic  repre- 
sentation of  the  location  of  the  factors  is  a  type  of  representation  common 
to  every  set  of  phenomena  which  can  be  expressed  as  percentages"  (Trow 
1916),  these  hypotheses  are  of  great  value,  for  by  aiding  in  the  correla- 
tion of  the  facts  of  inheritance  they  serve  to  increase  the  number  of 
observed  phenomena  statable  in  terms  of  order;  and  the  reduction  of 
experience  to  order  and  the  statement  of  this  order  in  simple  formulae, 
together  with  the  search  for  new  truth,  constitute  the  principal  tasks 
of  science.  If  this  work  of  correlation  has  been  well  done  the  whole 
body  of  facts  can  readily  be  placed  under  another  theoretical  interpre- 
tation and  described  in  a  new  set  of  terms  should  occasion  require. 

Although  it  may  be  that  the  chiasmatype  hypothesis  of  linkage  is  in 
certain  points  inadequate,  mathematically  (Trow  1916)  or  otherwise,  it  is 
nevertheless  true,  as  we  have  already  seen,  that  it  fits  the  case  very  well. 
At  the  same  time  we  may  remind  ourselves  that  the  fact  that  a  hypothesis 
works  well  is  no  guarantee  of  its  ultimate  truth.  But  even  if  the  chiasma- 
type interpretation  should  have  to  be  ever  so  greatly  modified  as  new 
facts  accumulate,  it  is  scarcely  to  be  doubted  that  the  chromosome  theory 
of  heredity  in  some  form  will  turn  out  to  be  in  accord  with  the  truth. 
With  respect  to  this  general  theory  Wilson  (1909)  writes  as  follows: 

'I  stand  with  those  who  have  followed  Oscar  Hertwig  and  Strasburger  in 
assigning  a  special  significance  to  the  nucleus  in  heredity,  and  who  have  recog- 
nized in  the  chromatin  a  substance  that  may  in  a  certain  sense  be  regarded  as  the 
idioplasm.  This  view  is  based  upon  no  single  or  demonstrative  proof.  It 
rests  upon  circumstantial  and  cumulative  evidence,  derived  from  many  sources. 
The  irresistible  appeal  which  it  makes  to  the  mind  results  from  the  manner  in 


LINKAGE  97 

which  it  brings  together  under  one  poinl  of  view  a  multitude  of  facts  thai  other- 
wise remain  disconnected  and  unintelligible.  What  arrests  the  attention  when 
the  facts  are  broadly  viewed  is  the  unmistakable  parallel  between  the  course  of 
heredity  and  the  history  of  the  chromatin-substance  in  the  whole  cycle  of  it- 
transformation.  In  respect  to  some  of  the  mosl  important  phenomena  of 
heredity  it  is  only  in  the  chromatin  thai  surh  a  parallel  can  be  accurately  traced. 
It  is  this  substance,  in  the  form  of  chromosomes,  thai  Bhows  the  association  of 
exactly  equivalent  maternal  and  paternal  elements  in  the  fertilization  of  the 
egg.  In  it  alone  do  we  clearly  see  the  equal  distribution  of  these  elements  to 
every  part  of  the  body  of  the  offspring.  In  the  perverted  forma  of  development 
that  result  from  double  fertilization  of  the  egg  and  the  like  it  is  only  in  the 
abnormal  distribution  of  the  chromatin-substance  by  multipolar  division  thai  we 
see  a  physical  counterpart  of  the  derangement  of  development.  Only  in  the 
chromatin-substance,  again,  do  we  see  in  the  course  of  the  maturation  of  the 
eerm  cells  a  redistribution  of  elements  thai  shows  a  parallel  to  the  astonishing 
disjunction  and  redistribution  of  the  factors  of  heredity  that  are  displayed  in  the 
Mendelian  phenomenon." 

With  more  particular  reference  to  the  chiasmatype  hypothesis  Wilson 
(1913)  says: 

"This,  admittedly,  is  a  bold  venture  into  a  highly  hypothetical  region.  It- 
justification  is  the  pragmatic  one  that  it  'works.'  The  hypothesis  gives  us  the 
only  intelligible  explanation  that  has  yet  been  offered  for  a  series  of  undoubted 
facts;  and  it  is  certainly  worthy  of  the  most  attentive  further  examination 
We  have  much  to  gain  and  nothing  to  lose  by  the  use  of  explanatory  hypoth< 
that  are  naturally  suggested  by  the  facts  and  help  us  to  formulate  them  for 
analysis,  so  long  as  such  hypotheses  are  not  allowed  to  degenerate  into  dogmas 
accepted  as  an  act  of  faith,  but  are  only  used  as  instrument- tor  further  observa- 
tion and  experiment." 

Bibliography  at  end  of  Chapter  XVIII. 


CHAPTER  XVIII 
WEISMANNISM  AND  OTHER  THEORIES 

The  theory  of  heredity  described  in  the  foregoing  chapters,  though 
resting  on  its  own  foundation  of  observational  and  experimental  evidence, 
shows  in  some  of  its  features  the  influence  of  certain  earlier  speculative 
hypotheses,  particularly  those  set  forth  by  Weismann.1  Some  of  the 
conceptions  embodied  in  these  hypotheses  are  consequently  involved  in 
cytological  and  genetical  discussions  of  the  present  day,  and  for  this 
reason  we  shall  here  outline  their  main  points,  briefly  indicating  wherein 
our  modern  theory  has  advanced  beyond  them. 

Although  conceptions  of  other  types  arose  very  early,  many  of  the 
hypotheses  in  question  were  based  on  the  assumption  that  the  phenomena 
of  heredity  and  development  are  the  result  of  the  activity  of  ultimate 
living  particles  of  ultramicroscopic  size.  Thus  Herbert  Spencer  (1864) 
built  up  a  theory  of  considerable  proportions  about  his  'physiological 
units/  and  these  formed  the  prototype  of  the  units  postulated  in  many 
later  theories.  Of  these  theories  the  most  prominent  were  those  of 
Darwin,  Nageli,  de  Vries,  and  Weismann. 

Darwin's  Hypothesis  of  Pangenesis. — In  his  Variations  of  Plants  and 
Animals  under  Domestication  (1868)  Darwin  included  a  chapter  on  his 
" Provisional  Hypothesis  of  Pangenesis,"  which,  though  offered  only  as  a 
suggestion,  excited  great  interest  in  the  field  of  biology,  especially  after 
the  advances  made  in  cytology  a  few  years  later.  In  several  points  it 
closely  resembled  a  theory  propounded  by  Buffon  more  than  a  century 
earlier  (1749).  Darwin  clearly  saw  in  the  cytological  aspects  of  heredity 
one  of  the  great  biological  problems  of  the  future,  but  his  only  specula- 
tions on  the  subject  were  embodied  in  the  pangenesis  hypothesis,  which 
may  be  stated  as  follows : 

All  the  cells  of  the  organism  at  all  stages  of  development  give  off  small 
particles,  or  gemmules,  which  multiply  by  fission  and  circulate  throughout 
all  parts  of  the  body.  These  gemmules  pass  to  the  germ  cells,  carrying 
with  them  the  power  to  reproduce  cells  like  those  from  which  they  came. 
In  this  way  units  representing  all  the  kinds  of  cells  composing  the  organ- 
ism are  collected  in  the  gametes  (or  spores  or  buds)  and  are  thus  passed 
on  to  the  next  generation.  During  the  embryogeny  of  the  new  individual 
the  gemmules  are  so  distributed  that  at  the  proper  times  and  places  they 

1  See  Kellogg  (1907),  Delage  and  Goldsmith  (1913),  Thomson  (1899,  1913)    and 
Conklin  (1915). 

398 


WEISMANNISM  AND  OTHER  THEORIES  399 

develop  into  cells  like  those  from  which  they  originally  migrated,  in 
this  manner  building  up  a  new  individual  like  t  lie  parent.  Some  of  the 
gemmules  do  not  function  until  a  comparatively  late  stage  in  the  onto- 
geny, and  others  may  remain  latent  through  several  generations:  on 
these  two  assumptions  it  is  possible  to  account  for  the  late  appearance  of 
certain  characters  and  for  the  fact  that  others  may  "skip"  one  or  more 
generations.  It  is  further  supposed  that  some  gemmules  remain  Litem 
in  the  individuals  of  one  sex:  thus,  for  instance,  the  characters  normally 
present  only  in  the  male  may  be  transmitted  through  the  female. 

In  the  many  criticisms  of  this  hypothesis  the  tendency  has  been  to 
judge  and  condemn  it  solely  on  the  ground  of  the  supposed  transport  ation 
of  the  gemmules  from  the  body  cells  to  the  germ  cells,  for  which  no  direct 
evidence  has  ever  been  discovered.     It  should  not  be  forgotten,  however, 
that  Darwin  suggested  an  explanation  for  the  phenomena  of  heredity 
on  the  basis  of  representative  material  units  in  the  cells,  a  conception 
which  was  of  the  greatest  importance  in  that  it  constituted  the  starting 
point  for  later  fruitful  investigations  and  theories.     The  migration  of 
the  gemmules  was  postulated  largely  to  account  for  the  phenomena  of 
regeneration   and   the   inheritance   of   acquired   somatic    modification-. 
Since  regeneration  may  be  explained  as  well  on  other  grounds,  and  Bince 
the  evidence  for  the  inheritance  of  acquired  somatic  modification-  ifl 
for  the  most  part  of  such  extremely  doubtful  value,  such  a  migration  of 
representative  units,  first  denied  by  Galton  (1875),  has  come  to  be  re- 
garded as  unnecessary.     A  theory  postulating  representative  particles 
but  no  such  migration  was  that  of  de  Vries. 

De  Vries's  Theory  of  Intracellular  Pangenesis.— According  to  de 
Vries  (1889)  the  particles  of  hereditary  substance,  or  pangens,  do  not 
represent  different  kinds  of  cells  as  Darwin  thought,  but  stand  rather  for 
different  elementary  characters  or  qualities  out  of  which  the  many  visible 
characters  of  the  organism  are  built  up.  Furthermore,  these  living 
elements  or  pangens  do  not  pass  from  cell  to  cell,  but  merely  circulate 
between  the  nucleus,  where  a  complete  outfit  of  them  is  conserved,  and 
the  other  parts  of  the  cell— hence  the  term  "intracellular  pangenesis." 
In  this  way  the  characters  brought  into  the  new  individual  through  the 
nucleus  are  delivered  to  the  cell  as  a  whole.  Contrary  to  the  idea  of 
Darwin  and  especially  to  that  of  Weismann  (see  below),  all  the  cells 
nuclei)  of  the  body  contain  pangens  for  all  the  hereditary  characters: 
they  are  not  sorted  out  as  development  proceeds. 

Nageli's  Idioplasm  Theory-  A  highly  speculative  theory  of  a  some- 
what  different  type  was  that  formulated  by  Nfigeli  (1884)  five  years 
before  that  of  de  Vries.  Protoplasm  was  thought  by  Nfigeli  to  be  made 
upofavast  number  of  fundamental  living  units;  these  he  called  micella. 
As  a  result  of  the  ways  in  which  these  molecular  complexes  or  micellae 
may  be  arranged,  there  are  in  the  cell  two  kinds  of  protoplasm:  m  nutri- 


400  INTRODUCTION  TO  CYTOLOGY 

tive  protoplasm,  or  trophoplasm,  the  micellae  have  no  regular  orienta- 
tion, whereas  in  idioplasm  they  are  oriented  in  a  particular  manner. 
According  to  Nageli  the  phenomena  of  heredity  are  due  to  the  constitu- 
tion and  transmission  of  this  idioplasm;  idioplasm  is  the  physical  basis  of 
inheritance.  It  is  not  confined  to  the  nucleus,  but  forms  an  elaborately 
constituted  network  extending  throughout  all  the  cells  of  the  organism. 
By  arranging  themselves  in  various  groupings  within  this  network  the 
micellae  are  able  to  bring  about  the  development  of  the  many  specific 
characters.  The  further  details  of  this  highly  "fragile"  hypothesis  are 
summarized  in  convenient  form  by  Delage  and  Goldsmith  (1913),  who 
point  out  that  in  spite  of  its  many  unsupported  assumptions  it  did  involve 
two  fertile  ideas:  first,  that  there  are  two  kinds  of  protoplasm,  one  of 
which  carries  the  characters  of  the  organism;  and  second,  that  there  are  a 
limited  number  of  elementary  characters  which  combine  in  various  ways 
to  produce  the  many  visible  characters. 

Weismann's  Theory. — The  most  highly  developed  and  influential  of 
all  such  speculative  theories  was  that  of  Weismann.  On  the  basis  of  the 
conception  of  pangens  Weismann  built  up  the  highly  involved  system 
of  hypotheses  set  forth  in  his  Das  Keimplasma  of  1892  and  in  more 
elaborated  form  in  his  Evolution  Theory  of  1902.  Certain  modifications 
were  later  made. 

As  Delage  and  Goldsmith  have  noted,  Weismann  incorporated  in  his 
theory  several  of  the  stronger  points  of  earlier  theories,  such  as  Darwin's 
conception  of  representative  particles,  Nageli's  elementary  characters, 
and  de  Vries's  intracellular  migration  of  particles.  With  Nageli  he  dis- 
tinguished between  nutritive  morphoplasm  and  hereditary  idioplasm  or 
germ-plasm,  but  unlike  Nageli  he  identified  the  idioplasm  with  the 
chromatin  of  the  nucleus.  His  conception  of  the  constitution  of  the 
idioplasm  was  essentially  as  follows: 

The  ultimate  unit  in  all  living  matter  is  the  biophore,  which  is  a 
minute  living  particle  capable  of  growth  and  reproduction — a  vital  unit. 
The  many  kinds  of  biophores  in  a  given  cell  represent  the  many  characters 
of  that  individual  cell:  they  are  not  bearers  of  the  characters  as  such 
(though  Weismann  often  spoke  of  them  in  this  fashion),  but  are  rather 
factors  upon  whose  presence  the  development  of  the  characters  depends. 
The  biophores  are  grouped  to  form  vital  units  of  a  higher  order,  known  as 
determinants.  The  determinant,  since  it  is  composed  of  the  many  kinds 
of  biophores  in  the  cell,  has  the  power  of  determining  the  development  of 
a  certain  type  of  cell  or  group  of  cells.  In  general,  therefore,  there  are  as 
many  sorts  of  determinants  in  the  organism  as  there  are  types  of  cells, 
or  " independently  variable  parts,"  to  be  developed.  The  determinants 
are  in  turn  grouped  into  ids.  A  single  id  contains  all  the  kinds  of  deter- 
minants, and  so  stands  for  the  sum  of  all  the  characters  of  the  organism: 
it  contains  the  "complete  architecture'1   of  the  germ-plasm.     The  ids 


WEISMANNISM  AND  OTHER  THEORIES  101 

in  a  given  species  differ  only  slightly  among  themselves,  the  differences 
corresponding  to  the  variations  observed  within  the  specie.-:  they  are  the 
" ancestral  germ-plasms "  which  have  been  contributed  by  past  genera- 
tions. The  ids  are  identified  with  the  visible  chromatin  granules  in  the 
nuclear  reticulum  or  in  the  chromatin  thread  during  mitosis.  In  mosl 
cases  the  ids  are  grouped  to  form  idants}  or  chromosomes.  In  some  forme 
which  have  a  large  number  of  granular  chromosomes  it  is  possible  thai 
each  is  composed  of  but  one  id.  The  id  therefore,  rather  than  the 
chromosome,  is  the  unit  of  primary  importance.  In  case  there  are 
several  ids  in  a  chromosome  (idant)  they  are  arranged  in  a  Linear  serii 
The  idea  that  the  chromosomes  are  all  alike  since  they  carry  closely  -mil- 
iar ids  was  later  (1913)  modified  by  Weismann,  largely  as  the  result  of 
the  demonstration  that  very  minute  characters  are  segregated  in 
Mendelian  fashion. 

With  the  aid  of  this  elaborate  mechanism  Weismann  explained  onto- 
genetic development  in  the  following  manner.  In  the  fertilized  <'<*¥.  from 
which  the  individual  is  to  develop  all  the  kinds  of  determinants  are 
present:  thoes  of  the  female  parent  are  contained  in  the  egg  nucleus 
and  those  of  the  male  parent  are  brought  in  by  the  nucleus  of  the 
spermatozoon.  During  the  long  series  of  cell-divisions  beginning  with 
the  fertilized  egg  and  ending  with  the  completion  of  the  mature  organism, 
the  many  kinds  of  determinants  are  sorted  out  through  a  progressive 
disintegration  of  the  ids,  and  are  distributed  in  a  definite  and  orderly 
manner  to  the  different  parts  of  the  body.  Many  somatic  mitoses  are 
therefore  regarded  not  as  equational  (erbgleich),  but  in  reality  qualitative 
(erbungleich) .  When  a  given  determinant  finally  reaches  the  proper  cell, 
i.e.,  when  that  cell  is  finally  formed,  the  determinant  splits  up  into  its 
constituent  biophores;  and  these,  through  their  action  upon  the  cell 
elements,  give  to  the  cell  its  specific  characters.  The  general  character 
of  a  cell  is  accordingly  due  to  the  type  or  types  of  determinant  which 
it  receives.  For  Weismann,  therefore,  development  (ontogenesis)  w&s 
definitely  bound  up  with  the  evolution  or  unfolding  of  a  complex  struc- 
ture contained  in  the  fertilized  egg.  Although  he  did  not  hold  thai 
the  units  in  the  egg  have  the  same  spatial  relations  as  (heir  corresponding 
characters  or  structures  in  the  adult,  it  has  been  said  with  some  degree 
of  truth  that  he  transferred  preformationism  to  the  nucleus. 

Such  being  Weismann's  conception  of  development ,  how  did  he  account 
for  heredity?  If  the  various  kinds  of  body  cells  in  an  individual  are 
characterized  by  different  types  of  determinants,  how  is  it  that  the  germ 
cells,  or  gametes  and  fertilized  egg  through  which  this  individual  is 
to  give  rise  to  the  next  generation,  possess  a  complete  outfit  of  deter- 
minants? According  to  Darwin's  hypothesis,  outlined  in  the  foregoing 
pages,  representative  particles  or  gemmules  are  contributed  by  all  the 
body  cells  at  all  stages  to  the  germ  cells,  by  which  they  are  transmitted 

L'fl 


402 


INTRODUCTION  TO  CYTOLOGY 


to  the  next  generation.  (See  Fig.  157,  A.)  Such  a  contribution  of 
elements  from  the  body  cells  to  the  germ  cells  was  denied  completely  by 
Weismann.  He  held  rather  that  a  certain  portion  of  the  complete  germ- 
plasm  (idioplasm;  chromatin)  of  the  fertilized  egg  is  carried  along  un- 


Fig.  157. — Diagram  illustrating  the  hypotheses  of  Darwin  and  Weismann.  The  large 
circles  represent  successive  generations  of  individuals,  and  the  small  circles  their  germ 
cells.  For  the  sake  of  simplicity  inheritance  is  shown  as  uniparental  rather  than  biparental. 
A,  Darwin's  Hypothesis  of  Pangenesis.  The  branching  solid  lines  ending  in  arrows  repre- 
sent the  sorting  out  of  the  hereditary  units  (gemmules)  during  ontogenesis;  the  dotted 
arrows  show  the  migration  of  gemmules  from  the  body  cells  to  the  germ  cells,  by  which 
they  are  carried  into  the  next  generation.  B,  C,  Weismann's  theory  of  the  continuity  of 
the  germ-plasm,  with  no  contribution  of  hereditary  units  from  the  body  cells  to  the  germ 
cells.  In  one  case  (B)  the  germ  cells  are  set  aside  at  the  beginning  of  ontogenesis,  and  in 
the  other  (C)  much  later.  In  both  cases  the  "complete  germ-plasm"  is  delivered  to  the 
germ  cells  through  a  shorter  or  longer  series  of  equational  divisions  (heavy  lines). 

changed  and  delivered  intact  to  the  germ  cells.  It  had  been  shown 
(Haeckel  1874;  Rauber  1879;  Jaeger  1878;  Nussbaum  1880;  Galton) 
that  in  certain  animals  the  primitive  germ  cells  are  set  aside  at  once  when 
development  begins,  and  Weismann  pointed  out  that  they  are  therefore 
differentiated  before  any  sorting  out  of  the  hereditary  units  can  have 


WEISMANNISM  AND  OTHER  THEORIES  in:; 

taken  place.  Hence  the  germ  cells  are  really  produced  by  the  germ  cells 
of  the  previous  generation  and  not  by  the  individual's  own  soma  body) 
at  all:  they  are  present  from  the  beginning  of  development  with  the  full 
hereditary  outfit,  and  by  a  few  equational  divisions  they  give  rise  to  the 
gametes.    This  is  represented  in  Fig.  157,  B.     En  the  more  usual  of 

those  animals  and  plants  in  which  the  germ  cells  appear  later  in  the  onto- 
geny Weismann  held  that,  although  a  sorting  out  of  the  units  occurs  in  the 
majority  of  the  cells  during  ontogenesis,  those  meristematic  cells  which 
constitute  the  chain  connecting  the  fertilized  egg  with  the  germ  cells 
the  germ  track  (Keimbahri) — maintain  the  undiminished  germ-plasm 
(Fig.  157,  C).  Thus  in  this  case  as  in  the  other  there  is  a  continuity  of 
the  germ-plasm,  if  not  a  continuity  of  the  germ  cells  (unless  meristematic 
cells  also  be  regarded  as  germ  cells).  Since  the  germ-plasm  of  any 
generation  is  derived  directly  from  that  of  the  preceding  one.  it  is  continu- 
ous through  an  unlimited  number  of  generations;  and  the  successive  somas 
(bodies)  are,  so  to  speak,  side  branches  given  off  at  intervals  from  the  main 
stream  of  the  germ-plasm. 

In  elaborating  the   above   views    Weismann    (1885,    1892)    insisted 
strongly  upon  the  independence  of  the  "potentially  immortal"  germ- 
plasm  and  the  transient  and  mortal  soma1.     He  argued  that  since  there 
is  no  contribution  of  hereditary  elements  from  the  soma  to  the  germ  cells, 
somatic    changes    being  in  no  way  impressed    upon  the  germ  cells  from 
which  the  next  generation  is  to  arise,  there  can  be  do  inheritance  of 
acquired    somatic    modifications.     In    multicellular    animals    the    only 
inherited  variations  are  those  originating  in  the  germ-plasm  of  the  germ 
cells  or  germ  track  as  responses  to  internal  (nutritive  etc.)  or  external 
environmental  stimuli,  or  as  the  result  of  recombinations  of  hereditary 
units  at  the  time  of  fertilization   (amphimixis).     Weismann  admitted 
that  the  germ-plasm,  though  remarkably  stable,  might  be  altered  directly 
by  the  environment  or  even  by  modifications  in  the  surrounding  soma; 
but  he  denied  that  in  the  latter  case  the  alteration  would  be  of  such  a 
nature  as  would  cause  the  reappearance  of  the  same  somatic  modifica- 
tion in  the  next  generation.     With  Weismann.  as  with  Mendel,  the  main 
problem  of  heredity  was  not  to  discover  how  the  characters  of  the  organ- 
ism get  into  the  germ  cells  which  it  produces,  but  rather  how  the  char- 
acters of  an  organism  are  represented  in  the  germ  cell  from  which  it  is  pro- 
duced (Darbishire  1911,  Chapter  12).     He  attempted  to  show  how  it  is 
that  the  stream  of  germ-plasm  on  the  one  hand  maintains  a  stability  suffi- 
cient to  account  for  the  resemblance  be1  ween  t  he  successr  e  bodies  spring- 
ing from  it  at  intervals,  and  on  the  other  hand  undergoes  orderly  chang 
responsible    for  the   evolutionary    advance   shown   in  a  long  series  of 
generations.     Jn  the    words    of   Agar   (Bower,    Kerr,    and    Agar,    1919  . 
"According  to  Darwin,    parents  truly   transmit   their  characteristics  to 

1  See  discussion  of  senescence  in  Chapter  VII. 


404  INTRODUCTION  TO  CYTOLOGY 

their  offspring  (by  means  of  the  gemmules).  According  to  the  modern 
view  [Mendel;  Galton;  Weismann],  however,  children  resemble  their 
parents  not,  strictly  speaking,  because  the  latter  have  passed  something 
on  to  them,  but  because  both  have  been  produced  from  the  same 
germ-plasm"  (p.  91).  "The  parent  is  rather  the  trustee  of  the  germ- 
plasm  than  the  producer  of  the  child"  (Thomson  1913). 

Weismann  attempted  further  to  account  for  the  variations  effective 
in  evolution  on  the  basis  of  his  theory  of  Germinal  Selection.  He  sup- 
posed that  the  determinants,  while  multiplying  in  the  germ  cells,  are 
subject  to  selection  like  all  other  organic  units.  Some  determinants, 
being  better  placed  with  respect  to  the  nutritive  conditions,  are  favored 
thereby  and  grow  stronger  and  more  influential,  while  others  undergo 
changes  in  the  opposite  direction.  The  cells  or  parts  of  the  organism 
receiving  the  determinants  which  have  had  the  advantage  in  the  struggle 
become  better  developed  than  those  receiving  the  weaker  determinants. 
As  this  process  continues  from  generation  to  generation  the  new  variation 
gradually  increases  until  it  becomes  pronounced  enough  to  be  laid  hold  of 
by  natural  selection.  In  this  manner  Weismann  accounted  for  the 
preservation  of  small  variations  not  yet  of  selective  value,  and  for 
continued  variation  along  definite  lines  (orthogenesis)  in  both  plus  and 
minus  directions.  Thus  for  him  selection  was  the  cardinal  principle 
which  ruled  not  only  over  organisms,  but  also  over  cells,  ids,  deter- 
minants, and  biophores.  As  he  himself  stated  it,  "This  extension  of  the 
principle  of  selection  to  all  grades  of  vital  units  is  the  characteristic 
feature  of  my  theories." 

Some  Modern  Aspects  of  Weismannism. — Although  the  distinction 

between  soma  cells  and  germ  cells  is  not  now  drawn  so  sharply  as  in  the 

days  of  Weismann,  it  is  nevertheless  of  interest  to  note  certain  facts 

adduced  in  support  of  his  contention  that  the  germ-plasm  is  continuous. 

In  Ascaris  megalocephala   (Boveri   1887c,    1889,    1891,    1892,    1904; 

Zacharias  1913)  it  is  observed  that  at  the  second  cleavage  mitosis  the 

chromsomes  in  one  blastomere  remain  entire,  while  in  the  other  blastomere 

they  become  broken  up  into  smaller  pieces,  some  of  which  are  lost  in  the 

cytoplasm   and  are  not  included  in  the  daughter  nuclei  (Fig.  158,  A). 

This  process  is  called  "  chromatin  diminution."     At  the  third  and  fourth 

cleavage    mitoses  a  similar  diminution   occurs  in  all  the  blastomeres 

but  one ;  in  this  one  the  chromosomes  remain  entire.     At  the  fifth  division 

it  is  seen  that  in  the  one  undiminished  cell  no  further  diminution  occurs 

as  it  divides,  and  its  descendants  become  the  germ  cells.     The  primary 

germ  cell  is  therefore  set  apart  at  the  fourth  mitosis;  and,  whereas  the 

other  embryonic  cells  giving  rise  to  somatic  structures  have  undergone  a 

diminution,  the  entire  chromatin  outfit  is  delivered  to  the  germ  cells 

through  the  undiminished  cells  of  the  germ  track.     A  similar  condition 

is  present  in  Miastor  (Kahle   1908;  Hegner  1912,    1914).     In  Ascaris 


WEISMAXXISM  AXD  OTHER  THEORIES 


105 


canis  (Walton  1918)  the  germ  cella  are  similarly  sel  aside  at  the  seventh 
cleavage  mitosis. 

In  criticizing  this  supposed  evidence  for  the  independence  and  con- 
tinuity of  the  germ-plasm  Child  (1915)  points  out  that,  since  undi- 
minished cells  may  give  rise  to  other  cells  as  well  as  germ  cells  in  the  early 
divisions,  the  process  observed  may  represent  merely  a  segregation  of 
different  organs  rather  than  a  separation  of  the  germ-plasm  from  the 
soma;  and  that  the  non-diminution  of  the  chromatin  in  the  germ  track 
may  be  the  result  of  the  differentiation  of  the  germ  cells  rather  than  its 
cause,  the  differentiation  at  this  stage  being  primarily  a  physiological 


Fig.  158. 

A,  chromatin  diminution  in  Ascaris  megalocephala.  The  second  cleavage  mitosis  la  in 
progress:  all  the  chromatin  is  retained  in  the  upper  blastomere,  from  which  the  germ 
cells  are  to  arise,  whereas  chromosome  diminution  occurs  in  the  lower  blastomere,  which 
is  to  give  rise  to  the  somatic  cells.  (After  Boveri.)  B,  third  nuclear  division  in  the  y<  t 
unsegmented  egg  of  Chironomus  confinis  showing  the  early  setting  aside  of  the  primitive 
germ  cells  at  the  lower  end.     (After  Hasper.) 

(metabolic)  one.  He  refers  to  certain  later  researches  of  Boveri  1910), 
which  apparently  show  that  "the  occurrence  or  non-occurrence  of  chro- 
matin diminution  in  a  nucleus  depends,  not  upon  its  qualitative  con- 
stitution, but  upon  its  cytoplasmic  environment."  Prom  this  il  is 
concluded  that  "the  'germ  path'  is  a  feature  of  the  cytoplasm,  and  the 
cytoplasm  is  not,  properly  speaking,  a  part  of  the  germ-plasm  al  all. 
but  represents  the  soma  of  the  cell"  (p.  327). 

In  support  of  this  conclusion  we  may  cite,  as  does  Child,  those  cafi 
among  insects  (Hasper  on  Chironomus,  1911  ;  Begneron  Micutor,  1912 
1914)  and  copepods  (Haecker  1897,    L902;    Annua    1911)    in    which    the 
substance  ofjlic  future  germ  cells  may  be  distinguished  very  early  in 
the  embryogeny,  even  in  the  undivided  egg,  either  as  a  visibly  difTeren- 


406  INTRODUCTION  TO  CYTOLOGY 

tiated  region  of  the  cytoplasm  (Keimbahn-plasma)  (Fig.  158,  B),  or 
by  the  presence  of  certain  cytoplasmic  granules  or  inclusions  (Keimhahn 
determinants).  These  latter  are  ultimately  delivered  to  the  definitive 
germ  cells,  the  nuclei  at  the  same  time  showing  no  differences  in  the  germ 
and  soma  cells.  Although  such  cases  seem  to  show  that  "the  factors 
determining  what  shall  become  germ  cells  and  what  somatic  structures 
apparently  exist  in  the  cytoplasm  and  not  in  the  nuclei'1  (Child  1915, 
p.  329),  it  is  nevertheless  very  significant  for  the  chromosome  theory  of 
heredity  that  only  in  the  germ  cells,  whatever  the  cause  of  their  differ- 
entiation from  the  other  cells  of  the  body,  should  the  chromatin  be 
retained  in  the  complete  state  in  the  cases  of  Ascaris  and  Miastor. 
Whatever  may  be  the  relation  of  the  chromatin  to  differentiation,  and 
whatever  may  be  the  degree  of  its  independence  of  the  soma-plasm,  it 
is  noteworthy  that  here  it  is  precisely  in  the  germ  cells  and  in  the  cells  of 
the  germ  track — the  cells  especially  important  in  heredity — that  the 
chromatin  shows  an  unbroken  continuity  from  cell  to  cell  and  conse- 
quently from  generation  to  generation.  Were  the  chromosome  mechan- 
ism disturbed  in  these  cells  as  it  is  in  the  somatic  cells,  or  should 
" diminished"  cells  regenerate  a  completely  normal  organism,  a  serious 
obstacle  would  be  in  the  path  of  the  chromosome  interpretation  of  here- 
dity as  now  formulated.  The  actual  behavior  of  the  chromatin  in  the 
germ  track  of  Ascaris  argues  for  rather  than  against  the  chromosome 
theory,  at  least  as  regards  hereditary  transmission. 

A  much  used  argument  against  Weismann's  theory  of  development 
(ontogenetic  differentiation)  is  found  in  the  phenomenon  of  regeneration. 
It  is  well  known  that  in  certain  animals  and  especially  in  plants  a  portion 
of  the  body  consisting  solely  of  differentiated  cells  may  under  certain 
conditions  give  rise  to  a  complete  individual  with  functional  germ  cells. 
Weismann  accounted  for  such  regeneration  on  the  basis  of  an  additional 
hypothesis  which  stated  that  during  the  sorting  out  of  the  hereditary 
units  in  the  process  of  cell  differentiation  certain  "supplementary 
determinants"  are  carried  along  unaltered,  and  that  later,  if  occasion 
arises,  these  cause  the  development  of  the  differentiated  cells  into  an 
organism  with  all  the  usual  characters.  Since  in  certain  cases  (Begonia) 
almost  any  cell  of  the  body  may  undergo  regeneration  into  a  complete 
plant,  it  is  evident  that  all  of  the  body  cells  must  have  a  "complete" 
germ-plasm.  Hence  the  distinciton  between  a  germ-plasm  limited  to 
cells  capable  of  producing  an  entire  individual,  and  a  soma-plasm  present 
only  in  somatic  cells  without  such  power,  becomes  of  no  value.  Every 
cell  capable  of  regeneration — germ  cell,  meristem  cell,  or  differentiated 
somatic  cell — contains  the  complete  germ-plasm,  which  appears  to  be 
simply  the  chromatin  possessed  by  all  the  cells  alike.  Lack  of  power  to 
regenerate  is  not  due  to  a  lack  of  complete  germ-plasm  but  to  other 
conditions  associated  with  the  degree  of  differentiation  shown  by  the 


WEISMANNISM   AND  OTHER  THE0R1  107 

cells,  In  1  luis  using  the  terms  germ-plasm  :in<l  soma-plasm  (somato- 
plasm) synonymously  with  chromatin  and  cytoplasm  respectively, 
Weismann's  conception  of  the  chromatin  as  the  substance  especially 
important  in  heredity  remains,  although  his  theory  of  the  dependence 
of  ontogenetic  differentiation  upon  a  sorting  out  of  qualitatively  differ- 
ent units  of  this  substance  during  development  is  no  longer  held. 

This  use  of  the  term  germ-plasm  is  general  among  geneticists,  who 
are  concerned  with  the  problems  of  heredity,  and  may  be  distinguished 
from  that  of  certain  students  of  the  physiology  of  development,  by  whom 
germ-plasm  is  regarded  as  "any  protoplasm  capable,  under  the  proper 
conditions,  of  undergoing  regression,  rejuvenescence,  and reconstitution 
into  a  new  individual,  organism,  or  part"  (Child  1915,  p.  462).  From  this 
latter  point  of  view  the  germ-plasm  would  be  regarded  as  oil  her  the 
complete  protoplast  capable  as  acting  as  so  described,  or,  as  Child  is 
inclined  to  believe,  only  an  abstract  idea — merely  a  term  standing  for 
heredity. 

Weismann's  theory  of  the  sorting  out  of  hereditary  units  during  onto- 
genesis was  abandoned  not  only  because  of  irs  inapplicability  to  the 
results  of  certain  experiments,  but  also  because  no  support  for  it  °ould 
be  found  in  a  direct  study  of  the  cell  mechanism.  Strasburger  and  other 
investigators  insisted  strongly  that  so  far  as  can  be  ascertained  the 
division  of  the  chromatin  at  each  somatic  mitosis  is  exactly  equational, 
there  being  not  the  slightest  indication  of  such  a  difference  in  the  chin- 
matin  of  the  two  daughter  cells  as  might  be  expected  were  the  divisions 
qualitative  (erbungleich).  To  this  Weismann  had  only  to  reply  that 
since  the  differentiation  is  a  matter  not  of  ids  or  of  idants  but  of  determi- 
nants, the  two  nuclei  would  be  visibly  alike  in  spite  of  their  qualitative 
difference.  Although  certain  cases  have  been  described  in  which  growth 
is  not  equal  in  all  of  the  chromosomes  during  the  early  stages  of  develop- 
ment, and  although  the  two  daughter  nuclei  may  become  differentiated 
through  unequal  nutrition  after  their  formation,  as  Strasburger  suggested, 
most  biologists  have  adopted  the  view  that  all  of  the  somatic  nuclei  are 
qualitatively  alike  in  their  chromatin  content  so  far  as  its  hereditary 
powers  are  concerned.  They  have  thus  followed  de  Vries-  L889  in 
holding  that  factors  for  all  of  the  hereditary  characters  are  present  in 
all  of  the  somatic  cells,  a  conclusion  strongly  supported  by  the  facts  ol 
regeneration.  The  ontogenetic  differential  ion  of  the  cells  which  mani- 
fests itself  largely  in  cytoplasmic  changes,  as  well  as  the  relative  regen- 
erative powers  which  these  cells  possess,  arc  attributed  for  the  most  pari 
to  physiological  causes,  the  latter  in  large  measure  determining  what 
hereditary  capabilities  of  the  various  veils  shall  come  to  expression. 
The  distinction  between  the  view  of  Weismann  and  thai  of  more  recent 
investigators  is  made  clear  in  the  two  diagrams  of  Fig.  1~>(.>,  which  have 
been  copied  from   Conklin  (1919-1920). 


408 


INTRODUCTION  TO  CYTOLOGY 


Notwithstanding  the  fact  that  many  changes  have  been  made  in  its 
details,  Weismann's  theory  of  heredity  proved  to  be  of  much  greater  value 
than  his  theory  of  development,  Morgan  (Morgan  et  al.  1915,  pp.  223- 
227)  points  out  that  Weismann  made  three  contributions  to  the  study  of 
genetics,  which  may  be  stated  in  three  propositions:  (1)  The  germ-plasm 


GERtt   CE.LL 


V 

SOMATIC    CELLS 


Crthn  CELL 


SOMATIC    CELLS 


Fig.  159. — The  behavior  of  the  hereditary  units  in  ontogenesis  according  to  Weismann 
(A)  and  the  current  interpretation  (B).  In  A  the  determinants  in  the  nucleus  (-1,  2,  3,  4) 
are  supposed  to  be  distributed  differentially  to  the  various  somatic  cells.  In  B  the  genes 
(1,  2,  3,  4)  are  distributed  equally  to  every  cell,  but  the  cytoplasm  is  distributed  differen- 
tially. The  same  genes  working  upon  different  cytoplasms  produce  different  results  in 
various  somatic  cells.      {Diagrams  and  legend  from  Conklin,  1919-1920.) 

contains  independent  elements  which  may  be  substituted  one  for  another 
without  undergoing  change;  (2)  a  segregation  of  maternal  and  paternal 
factors,  pair  by  pair,  occurs  at  one  period  in  the  history  of  the  germ  cells; 
(3)  the  behavior  of  the  chromosomes  is  specifically  applicable  to  the 
problems  of  heredity.     In  these  principles  are  found  "the  basis  of  our 


WEISMANNISM  AND  OTHER  THEORIES  409 

present  attempt  to  explain  heredity  in  terms  of  the  cell,"  for  upon  them 
is  founded  the  Factorial  Hypothesis,  now  supported  by  a  large  mass  ol 
experimental  evidence. 

In  our  conception  of  the  uature  of  the  heredity  units  or  factors  we 
have  departed  widely  from  Weismann.  For  him  each  of  the  ids  arranged 
in  a  series  in  the  chromosome  represented  the  sum  of  the  characters  of  a 
complete  organism;  the  smaller  pails  were  represented  by  the  smaller 
units  (determinants)  composing  the  id,  and  these  units  in  turn  were  made 
up  of  biophores,  which  were  ultimate  and  independent  living  particles. 
According  to  our  modern  hypothesis  each  of  the  Berially  arranged  factors 
or  genes  exerts  an  influence  on  the  development  of  one  or  more  characters, 
but  does  not  stand  for  a  complete  organism  as  did  the  id,  or  for  a  pari 
of  it  as  did  the  determinant.  Moreover,  it  is  generally  regarded  as  a  mass 
of  some  complex  chemical  substance  whose  activities  are  due  to  its  defi- 
nite though  imperfectly  known  physico-chemical  properties,  rather  than 
to  forces  exerted  by  hypothetical  vital  units. 

In  justice  to  Weismann  it  should  be  pointed  out  that  the  frequently 
made  criticism  that  his  theory  was  a  vitalistic  one  is  warranted  only  to  a 
limited  extent.  Although  his  ultimate  hereditary  units,  the  biophoi 
were  regarded  as  actually  living  particles,  Weismann  stated  that  "they 
are  not  composed  in  their  turn  of  living  particles,  but  only  of  molecules, 
whose  chemical  constitution,  combination,  and  arrangement  are  such  as 
to  give  rise  to  the  phenomena  of  life."  He  was  careful  to  point  out  that 
in  spite  of  the  fact  that  it  cannot  be  proved  that  no  peculiar  vitalistic 
principle  exists,  we  should  hold  fast  to  a  purely  physico-chemical  basis 
of  life  "until  it  is  shown  that  it  is  not  sufficient  to  explain  the  tact-,  thus 
following  the  fundamental  rule  that  natural  science  must  not  assui in- 
unknown  forces  until  the  known  ones  are  proved  insufficient  ...  We 
can  quite  well  believe  that  an  organic  substance  of  exactly  proportioned 
composition  exists,  in  which  the  fundamental  phenomena  of  all  life 
combustion  with  simultaneous  renewal — must  take  place  under  certain 
conditions  by  virtue  of  its  composition"  (1902,  lecture  :>(>). 

The  manner  in  which  hereditary  factors  are  segregated  at  gameto- 
genesis  has  been  found  to  be  different  from  t  hat  conjeet  ured  by  Weismann. 
As  indicated  in  the  chapter  on  reduei  ion,  he  supposed  it  to  occur  t  hrough 
a  transverse  division  of  the  chromosome,  whereas  it  is  now  known  thai 
it  is  accomplished  by  the  disjunction  of  pairs  of  entire  chromosomes, 
the  separating  members  of  each  pair  being  qualitatively  different.  The 
"reduction"  predicted  by  Weismann  was  found  to  occur,  but  not  in  the 
manner  he  supposed.  As  shown  above,  his  idea  of  a  further  qualitative 
segregation  of  units  of  a  lower  order  in  the  somatic  divisions  has  Q01 
been  substantiated.  Notwithstanding  the  abandonment  of  his  theory 
of  development  and  the  changes  made  in  his  theory  of  heredity,  Weis- 
mann's  influence  on  both  cytology  and  genetics  was  enormous,  largely 


410  INTRODUCTION  TO  CYTOLOGY 

because  of  his  emphasis  upon  the  need  for  careful  studies  of  the  cell 
mechanism  at  the  critical  stages  of  the  life  history,  and  upon  the  idea 
that  this  mechanism  is  in  some  way  bound  up  with  the  phenomena  of 
heredity.  "It  has  been  Weismann's  great  service  to  place  the  keystone 
between  the  work  of  the  evolutionists  and  that  of  the  cytologists,  and 
thus  bring  the  cell-theory  and  the  evolution-theory  into  organic  con- 
nection" (Wilson  1900,  p.  13). 

We  may  further  point  out,  with  Morgan  (1915),  that  the  factorial 
hypothesis  assumes  only  three  things  about  the  factors:  they  are  constant, 
they  are  usually  in  duplicate  in  each  body  cell  and  immature  germ  cell, 
and  they  usually  segregate  in  the  maturing  germ  cells.  The  hypothesis, 
and  the  Mendelian  theory  in  general,  therefore  have  to  do  only  with 
heredity :  they  do  not  attempt  to  explain  the  causes  of  development.  They 
seek  rather  to  account  for  the  initial  resemblances  or  differences  in  here- 
ditary potentiality  which  are  observed  to  exist  between  the  germ  cells 
from  which  successive  generations  arise.  Between  the  materials  com- 
posing the  initial  factors  and  the  fully  expressed  characters  of  the  or- 
ganism "lies  the  whole  world  of  embryonic  development,"  to  which  the 
application  of  the  theories  under  consideration  has  not  yet  been  extended 
in  any  systematic  or  satisfactory  manner.  Nevertheless  many  investi- 
gators, though  realizing  the  failure  of  Weismann's  attempt  to  explain 
development  in  terms  of  representative  particles,  are  strongly  inclined 
to  the  view  that  since  the  Mendelian  characters  appearing  toward  ma- 
turity behave  as  though  associated  with  discrete  units  in  the  germ,  the 
course  of  ontogenetic  development  in  its  earlier  stages  must  also  be  due  in 
large  part  to  the  activity  of  factors  carried  by  the  nucleus.  Development 
is  thus  held  to  be  predetermined  or  controlled  by  an  internal  mechanism : 
external  agencies  act  only  by  affecting  the  operation  of  this  mechanism. 
The  factors  control  the  character  and  behavior  of  the  cells,  and  upon 
these  in  turn  the  organism,  which  is  a  cell  aggregate,  is  alone  dependent 
for  its  characters  and  activities.  In  place  of  the  early  hypothesis  on 
which  it  was  supposed  that  the  development  of  characters  is  controlled 
by  the  migration  of  determiners  or  pangens  from  the  nucleus  into  the 
cytoplasm  at  precisely  the  right  times  and  places,  we  now  have  the  theory 
that  the  factors  in  the  nucleus  probably  produce  their  effects  by  initiating 
series  of  chemical  reactions  which  involve  all  parts  of  the  cell.  As  Mor- 
gan (1920)  states,  "  Granting  that  differences  may  exist  in  the  nuclei  of 
different  species,  different  end  products  are  expected.  The  evidence 
that  such  differences  may  be  related  to  specific  substances  in  the  nucleus 
is  no  longer  a  speculation  but  rests  on  the  analytical  evidence  from  Men- 
delian heredity.  In  what  way  and  at  what  times  the  nuclear  materials 
take  part  in  the  determination  of  characters  we  do  not  know.  The 
essential  point  is  that  we  are  in  no  way  committed  to  any  interpretation. 
Stated  negatively  we  might  add  that  there  is  nothing  known  at  present 
to  preclude  the  possibility  that  the  influence  is  a  purely  chemical  process." 


1 1 '  E  ISM  A  X  Y  TSM  AND  OTH  E  R  Til  Et  >  R I ES  1 1  1 

Non-factorial  Theories.  -The  above  theory  <>!  the  dependence  <»!  the 
course  of  development  upon  the  operation  of  an  internal  factorial  mech- 
anism is  essentially  an    "elementalistic"   conception:   the   attempl    ie 

made  to  explain  the  organism  in  terms  of  its  constituenl  parts,  namely, 
the  cells  and  smaller  elements  contained  by  them.  A>  noted  in  our 
historical  sketch,  a  number  of  botanists  and  zoologists  many  yeara  ago 
called  attention  to  the  fact  that  limits  must  I"-  Bel  1<>  the  conception  «»t' 
the  cell  as  the  unit  of  struct  ure  and  function;  and  they  have  been  followed 
by  a  school,  made  up  largely  of  experimental  embryologists,  which  holds 
that  organization  is  not  the  result  of  cell  formation,  bu1  rather  precedes 
and  regulates  the  latter.  From  this  "organismar  standpoint  the  or- 
ganism as  a  whole,  and  not  one  or  another  of  its  elementary  parts,  is 
regarded  as  the  primary  individual.  This  individual  is  something  more 
than  the  cell  aggregate  pictured  by  Schleiden  and  Schwann:  it  dominates 
the  activity  of  its  constituent  members  from  the  beginning  of  the  life 
cycle  onward,  and  behaves  as  a  unit  irrespective  of  the  manner  and  degree 
of  its  subdivision  into  special  centers  of  action,  the  cells.  The  condition 
present  in  ccenocytic  plants  is  especially  noteworthy  in  this  connection, 
as  are  also  those  cases  among  animals  in  which  a  derangement  of  the 
early  embryonic  cells  does  not  prevent  the  eventual  attainment  of  the 
normal  form.  As  urged  with  much  force  by  Ritter  (1919),  "  the  organism 
in  its  totality  is  as  essential  to  an  explanation  of  its  elements  as  its 
elements  are  to  an  explanation  of  the  organism." 

•The  factorial  theory  may  also  be  said  to  represent  preformationism 
in  a  very  modern  form.     "We  are  sailing  nearer  the  preformation  coast 
than  at  any  time  since  the  modern  study  of  development  began  under 
von  Baer"    (Conklin  1913).     Directly  opposed  to  corpuscular  and  fac- 
torial theories  of  development  are  those  which  seek  to  explain  the  course 
of  ontogenesis  not  by  an  internal  mechanism  but  rather  as  the  result  ot 
the  influence  of  external  agencies  and  the  physiological  responses  Bhown 
by  protoplasm  in  the  form  of  cells  to  such  influence,  development    is 
held  to  be  truly  epigenetic.     The  control  exercised  by  environmental 
factors  during  the  organism's  early  developmental  stages,  and  the  effects 
of  various  tropisms  and  tactisms  between  the  component  cells  upon  the 
type  of  organization  resulting,  have  been  especially  emphasized  bj 
Hertwig,  Hartog,  Roux,  Herbst,  Driesch,  and  others.     The  most    sug- 
gestive recent  work  of  this  nature  in  plants  is  thai   of  Harper  (1908, 
1918o6)  on  colonial  alga*.     In  Hydrodictyon  and  Pediastrum  a  number  of 
free-swimming  cells  come  together  and  build  up  colonies  of  very  definite 
forms,  and  a  series  of  experiments  has  shown  that  the  position  in  the  colony 
of  any  given  cell  is  in  no  way  predetermined.     A>  already  pointed  out 
in  Chapter  XIV,  Harper  contends  i  hat  the  type  of  multicellular  organiza- 
tion thus  built  up  in  successive  life  cycles  is  to  be  explained  as  the  result 
of  physico-chemical  interactions  between  independent  cells  organized  as 


412  INTRODUCTION  TO  CYTOLOGY 

swarm  sixties,  and  nol    as  the  product  of  the  activity  of  a  system  of 
spatially  arranged  factors  in  a  special  germ-plasm. 

In  this  connection  the  name  of  Driesch  (1907-8,  1914)  has  become 
particularly  prominent,  not  only  because  of  his  great  experimental  ingen- 
uity, but  also  because  of  his  decision  that  the  facts  of  ontogenetic  devel- 
opment cannot  be  accounted  for  on  the  basis  of  any  mechanical  theory, 
either  now  or  in  the  future.  As  a  result  he  takes  the  unscientific  step  of 
assuming  the  existence  of  a  non-mechanical,  non-spatial,  non-psychic, 
non-energetic  "entelechy,"  which  presides  over  and  controls  develop- 
ment. Such  non-experiential  agencies,  manufactured  for  the  purpose  of 
solving  difficult  problems,  lead  to  experimental  indeterminism  and  tend 
only  to  obscure  the  points  at  issue :  they  may  furnish  convenient  names 
for  great  gaps  in  our  knowledge,  but  they  never  give  more  than  pseudo- 
explanations.  Nevertheless,  in  spite  of  his  tendencies  to  mysticism,  as 
Harper  (1919)  remarks,  Driesch  has  shown  the  impossibility  of  an  exact 
parallelism  in  spatial  configuration  between  the  germ-plasm  and  the 
multicellular  organism  as  a  whole:  there  can  be  no  strict  preformation 
in  development.  On  the  other  hand,  the  work  of  the Mendelians  shows 
clearly  that  development  cannot  be  completely  epigenetic:  nothing  seems 
clearer  than  that  development  is  at  least  in  part  dependent  upon  the 
orderly  operation  of  an  internal  organization  or  mechanism.  Wilson 
(1909,  pp.  106  ff.),  in  discussing  the  relation  of  the  chromatin  to  heredity 
and  development,  writes  as  follows: 

'But  do  we  really  need  to  employ  the  pangen  symbolism  in  the  consideration 
of  this  question?  It  seems  a  sufficient  basis  for  our  present  attack  on  the  problem 
to  assume  that  the  control  of  the  cell-activities  is  at  bottom  a  chemical  one  and  is 
effected  by  soluble  substances  that  may  pass  from  nucleus  to  protoplasm  and 
from  protoplasm  to  nucleus.  Certainly  it  is  to  such  a  view  that  very  many  of  the 
chemical  and  physiological  studies  in  this  field  are  now  unmistakably  pointing. 
The  opinion  is  gaining  ground  that  the  control  of  development  is  fundamentally 
analogous,  perhaps  closely  similar,  to  the  control  of  specific  forms  of  physiological 
action  by  soluble  ferments  or  enzymes  .  .  .  We  are  thus  led  to  something  more 
than  a  suspicion  that  the  factors  of  determination,  and  therefore  of  heredity, 
are  at  bottom  of  chemical  nature  .  .  .  The  conclusion  thus  becomes  highly 
probable  that  the  characteristic  differences  of  metabolism  between  different 
species,  including  those  involved  in  development,  are  traceable  to  initial  chemical 
differences  in  the  germ  cells.  In  so  far  as  the  chromatin  theory  expresses  the 
truth,  the  primary  basis  of  these  differences  may  be  sought  in  the  nuclear 
substance." 

A  Chemical  Theory  of  Heredity. — Among  the  theories  based  on  the 
conception  of  the  idioplasm  as  a  substance  with  a  special  chemical  consti- 
tution, rather  than  as  a  system  of  determinants,  may  be  mentioned  that 
of  Adami  (1908,  1918).  As  indicated  in  Chapter  III,  Adami  attributes 
the  phenomena  of  life  to  the  activities  of  a  protein-like  "biophoric  mole- 


WEISMANNISM  AND  OTHER  THE0RI1  s  II 

cule,"  which  is  made  up  of  a  chain  or  ring  of  amino-acid  radicles  to  which 
side-chains  of  various  kinds   may   become  attached,    \\nli   regard    to 
individual  development  it  is  supposed  thai    'in  the  ovum  there  is  one 
common  idioplasm  of  simple  type,  to  which,  when  distributed  in  the 
various  cells   derived   from  that    ovum,    different    side-chains    become 
attached,  according  to  the  relationships  assumed  by  those  cells,  so  thai 
the   cells   of  different  orders  arc  controlled  and   formed  around   proto- 
plasmic or  idioplasmic  molecules  composed  of  those  central  rings  plus 
varying  series  of  side-chains"  (p.  14.")).    With  Driesch  ii  is  held  thai    't  lu- 
st ructure  of  the  cells  in  a  multicellular  organism  is  a  function  of  their 
position,"  since ''the  position  of  the  cell  determines  the  modification  under- 
gone by  its  idioplasm."    Furthermore,  "  the  greater  t  lie  change  impressed 
upon  the  idioplasm  of  these  cells,  and  the  longer  that  idioplasm  is  sub- 
jected to  the  conditions  inducing  this  change,  the  more  permanently  will 
the  daughter  cells  exhibit  the  peculiar  alteration  in  the  idioplasm,  wit  h  con- 
sequent modified  structure  wherever  they  find  themselves  in  the  economy. 
We  have,  in  short,  to  recognize  that  two  orders  of  forces  determine  the 
structure  of  every  cell  in  the  body:  (1)  the  previous  influences  act  ingupon 
its  idioplasm  and  causing  it  to  be  of  a  particular  chemical  constitution; 
and  (2)  the  position  in  which  the  cell  finds  itself,  and  the  forces  acting 
momentarily  and  immediately  upon  its  idioplasm.     Or,  briefly,  these  two 
series  of  forces  are  inheritance  and  environment,  and  inheritance  and 
environment  determine  the  constitution  of  the  idioplasm  and  the  struc- 
ture of  the  cells"  (p.  151). 

"In  terms  of  this  theory,  therefore,  inheritance  essentially  d<  pends 
upon  the  chemical  constitution  of  the  idioplasm  or  the  life-bearing  or 
biophoric  protoplasm  of  the  germ  cells,  not  upon  the  number  of  the  sepa- 
rate ids  or  biophores  or  ancestral  plasms  or  pangens  contained  in  the  idio- 
plasm; and  variation,  whether  slight  and  individual,  or  extensive  and 
leading  to  the  production  of  new  species,  is  ultimately  the  expression  of 
modification  in  the  constitution  of  that  idioplasm  brought  about  by  envi- 
ronment. Whereas  Weismann's  theory  lays  stress  upon  relative  fixity 
in  the  constitution  of  the  idioplasm,  this  theory  admits  lively  the  capacity 
for  change  in  structure  of  the  same.  So  long  as  I  he  surrounding  condi- 
tions are  unaltered  the  idioplasm  is  unchanged;  alter  these  conditions  and 
the  idioplasm  is  liable  to  variation  in  constitution'    (pp.  152   3 

Adami  cites  certain  calculations  of  the  probable  size  of  inorganic  and 
organic  molecules  to  show  that  the  existence  of  a  system  of  determinants 
or  other  representative  particles  of  the  Weismannian  type  is  a  physicial 
impossibility.  He  also  points  out  thai  since  the  idioplasm  musl  increase 
enormously  in  bulk  by  the  addition  of  new  material  and  become  repeat- 
edly subdivided  as  ceils  and  individuals  multiply,  there  can  be  no  actual 
continuity  of  the  germ-plasm  through  countless  generations:  what  is 
eternal  is  rather  a  potential  continuity  of  molecular  arrangement    and 


414  INTRODUCTION  TO  CYTOLOGY 

constitution,  i.e.,  the  physical  and  chemical  properties  of  the  germ-plasm 

rather  than  the  substance  itself. 

Conclusion. — In  the  foregoing  pages  we  have  touched  upon  some  of 
the  most  important  biological  problems  toward  the  solution  of  which 
cytology  must  make  her  further  contributions.  With  regard  to  individual 
development  it  must  be  determined  on  the  one  hand  to  what  extent  the 
course  of  ontogenesis  is  dependent  upon  the  operation  of  an  internal  cell 
mechanism  and  how  this  mechanism  brings  about  its  results,  and  on  the 
other  hand  how  far  it  is  controlled  by  external  environmental  agencies: 
a  way  must  be  found  between  the  "Scylla  of  preformation  and  the 
Charybdis  of  epigenesis';  (Conklin  1913).  Furthermore,  the  manner 
and  the  causes  of  the  progressive  modification  of  the  hereditary  mechan- 
ism must  be  better  known  in  order  that  evolutionary  advance 
may  be  accounted  for.  With  respect  to  both  development  and  heredity 
the  roles  of  the  two  individualities,  the  cell  and  the  organism  as  a  whole, 
must  be  more  fully  ascertained  and  correlated. 

It  is  obvious  that  no  adequate  solution  of  any  of  these  problems  can 
be  reached  until  the  physico-chemical  constitution  of  protoplasm, 
especially  that  of  the  idioplasm  or  inheritance  material,  is  more 
clearly  disclosed.  Only  further  research  can  show  whether  we  shall 
continue  to  regard  the  idioplasm  or  chromatin  as  a  heterogeneous 
system  of  discrete  molecules  or  molecular  complexes  (factors  or  genes) 
with  a  definite  spatial  arrangement,  as  is  supposed  on  our  current 
Mendelian  theories,  or  shall  come  to  look  upon  it  as  a  single  enormously 
complex  chemical  substance  in  which  varying  side-chains  or  other  portions 
of  the  molecule  are  responsible  for  the  variety  of  results  observed.  It  is 
at  any  rate  a  striking  fact  that  ain  the  Mendelian  phenomenon  we  see  a 
synthesis,  splitting  apart,  and  recombination  of  determinative  factors 
that  is  singularly  like  that  of  chemical  elements  or  radicles"  (Wilson  1909, 
p.  108);  and  nothing  appears  more  clearly  evident  than  the  truth  of 
Wilson's  assertion  that  "  .  .  .in  the  union  of  cytology  and  biochemistry 
lies  our  greatest  hope  of  future  advance." 

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416  INTRODUCTION  TO  CYTOLOGY 

1909a.     Ueber  Beziehungen  des  Chromatins  zur  Geschlechtsbestimmung.     Sitzber. 

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1914.  Ueber  die  Charaktere  von  Echinidenbastardlarven  bei  verschiedenen  Men- 
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1918   (posthumous).     Zwei  Fehlerquellen  bei  Merogonieversuchen  und  die  Ent- 

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1912.  The  origin  of  maize.     Jour.  Wash.  Acad.  Sci.  2. 

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gvnodioischen  Pflanzen.     Ber.  Deu.  Bot.  Ges.  26a:  686-701. 
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Vererb.  1 :  291-329.     figs.  2. 


HEREDITY:  SEX  117 

10096.     Zur  Kenntnisa  der  Rolle  von  Kern  and  Plasma  beider  Vererbung.     [bid. 

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28 :  475-479.     figs.  3. 
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Bastardregeln.     Ergeb.  Anat.  u.  Entw.  16:  1    L40.     (Review. 
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14:171-174.     1  fig. 

27 


418  INTRODUCTION  TO  CYTOLOGY 

Galton,  F.     1875.     A  theory  of  heredity.     Jour.  Anthrop.  Inst. 
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1911.  Pollen  formation  in  (Enothera  gigas.     Ann.  Bot.  25:  909-940.     pis.  67-70. 

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Gates,  R.  R.  and  Thomas,  N.     1914.     A  cytological  study  of  (Enothera  mut.  lata 

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Goldschmidt,  R.     1911.     Einfuhrung  in  die  Vererbungswissenschaft.     Leipzig. 

1912.  Bemerkungen  zur  Vererbung  des  Geschlechtspolymorphismus.     Zeit.  Ind. 
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1916a.     Experimental    intersexuality    and    the    sex    problem.     Am.    Nat.    50: 

705-718. 
19166.     A  preliminary  report  on  further  experiments  in  inheritance  and  deter- 
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1917a.     A  further  contribution  to  the  theory  of  sex.     Jour.  Exp.  Zool.  22:   593- 

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Ohio  Jour.  Sci.  19:  409-410. 
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Seiler,  J.     1913,  1914.     Das  Verhalten  der  Geschlechtschromosomen  bei  Lepidop- 

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Ibid.  21:  127-102.     1  fig. 
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2. 
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424  INTRODUCTION  TO  CYTOLOGY 

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figs.  38. 
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1919.     The  ineffectiveness  of  oxygen  as  a  factor  in  causing  male  production  in 
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426  INTRODUCTION  TO  CYTOLOGY 

19056.     Studies  on  chromosomes.     II.  The  paired  microchromosomes,  idiochromo- 

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564. 

1906.  Studies  on  chromosomes.  III.  The  sexual  differences  of  the  chromosome 
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1909a.  Studies  on  chromosomes.  IV.  The  "accessory"  chromosome  in  Syro- 
mastes  and  Pyrrochoris  with  a  comparative  review  of  the  types  of  sexual  differ- 
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19096.  Studies  on  chromosomes.  V.  The  chromosomes  of  Metapodius.  A 
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1909c  The  cell  in  relation  to  heredity  and  evolution.  In  "  Fifty  Years  of  Darwin- 
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19125.     Studies  on  chromosomes.     VIII.  Observations  on  the  maturation-phenom- 
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Wilson,  E.  B.  and  Morgan,  T.  H.     1920.     Chiasmatype  and  crossing-over.     Am. 
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Wodsedalek,  J.  E.     1920.     Studies  on  the  cells  of  cattle  with  special  reference  to 
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21-38. 
19206.     Sex  intergradation  in  the  flowers  of  Mercurialis  annua.     Ibid.  7:  95-100. 

pi.  5. 
Zacharias,    E.     1913.     Die    Chromatin-Diminution   in    den    Furchungszellen    von 

Ascaris  megalocephala.     Anat.  Anz.  43:  33-53.     figs.  2. 
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INDKX 


Bold-face  numbers  indicate  pages  bearing  illustrations 

A 


Abies  (fir),  295 
Abraxas  (moth),  363 
Accessory  body,  88,  89 

chromosome,  358  ff. 
Acer  (maple),  159,  176,  236,  •_):'>,.>.  356 
Achromatic  figure,  66,  145 

in  animals,  111  ff. 

in  higher  plants,  17")  ff. 

operation  of,  182  ff. 

origin  of,  180 
Achromatin,  64 
Acids,  organic,  135 
Acquired  characters,  inheritance  of,  399, 

403 
Acrididse,  243 
Acrosome,  273,  275 
Actinophrys  (protozoan),  282 
Actinosphcerium  (protozoan),  211 
Activation  of  egg,  284  ff. 
Acton,  202,  205 
Adami,  50,  412  ff. 
Adiantum  (fern),  91,  122 
Adoxa  (angiosperm),  236 
Adsorption,  37 
JFAhalium  (slime  mold),  39 
Agar,  ix,  162,  403 
Agathis  (conifer),  295 
Agave  (century  plant),  134,  239 
Age,  136,  137,  138 
Akinetic  division,  143 
Albugo  (fungus),  289,  290 
Albumen  crystals,  135 
Albumin,  40,  41,  117 
Alchemilla  (angiosperm),  313,  315 
Aleurone,  134,  135,  333 
AlexiefT,  118 

Alicularia  (liverwort),  111 
Allelomorphic    pairs,     338    ff.,    378    ff. 
Allen,  C.  E.,  cell  plate,  190 

chromomeres,  155,  39  1 
plastids,  113 
reduction,  222,  236 
sex-determination,  357,  364  365 
spermatogenesis,  88,  89 


Allen,  R.  I ■'.,  91,  313,  31  l 
Mhum  (onion 

achromal i<-  figure,  l s- 

canalicul®,  48 

cell  wall,  L91 

chondriosomes,  116 

nucleolus,  65 

reduction,  234,  23d,  2 

somal i<-  chromosomes,  I  19,  l"-11 
AUolobophora  (annelid  worm  .  244, 
Altenburg,  391 

Alternation  of  general i<m~.  :;i  l 
Alt  man,  33,  40,  48,  115 
Alveolar  theory  of  protoplasm,  33 
Alveolation   of  chromosomes,    148,    149, 

242 
Alytes  (toad1.  237,  256 
.1  mblystegium  (moss  .  317 
Ambystoma  (salamander),  l»'i<> 
Amici,  6,  13 
Amides,  135 
Amitosis,  210  if. 

and  heredity,  212  215 

criticism  of  evidence,  213  215 
Amma,  405 

Amoeba  (protozoal,  42,  43,  44,  207,  222 
Amphiaster,  1  15,  177.  178,  286 
Amphibia,  239 
Amphimixis,  403 
.1  mphtoxus  (lancelel  \,  237 
Amphitene,  231,  2 
.1  mphiuma  (amphibian  I,  150 
Amygdalus  (angiosperm  I,  134 
Amylodextrin,  107 
Amyloplast,  104,  L06,  122 
Amylose,  1<>7 

\ii:il>a'iiili.  20  1 

Analysis  of  gametes,  2 
Anaphase 

heterotypic,  234,  236,  238,  239,  242 

somatic,  144,  1  r>.  1  17 
Anasa  (bug),  244,  245,  249 
Anastomoses,  I  17.  148,  1  19,  150 
Ancel,  256 
AncyracanthiM    (nematode    worm), 


367 


127 


428 


INDEX 


Anderson,  395 

Androcyte,  88,  89 

Androgone,  88,  89 

Aneura  (liverwort),  82,  87,  239 

Angiosperms 

fertilization,  298,  299 

sporogenesis,  225 
Antedon  (echinoderm),  327 
Antheridium,  87,  222,  289,  290 
Anthoceros  (liverwort) 

apical  cell,  27 

chondriosomes,  122 

fertilization,  293 

plastid,  103,  104,  105,  113,  114 

pyrenoid,  104,  108,  109 
Anthoryanin  pigments,  122,  135 
Anti-fertilizin,  287 
Antirrhinum  (snap  dragon),  332 
Aphis  (aphid),  318 
Apical  body,  89 
Apical  cell,  27 
Apical  growth,  27    . 
Apis  (bee),  318 

Apogamy,  311  ff.}  312  v 

Apospory,  311,  315  ff.,  316 
Apotettix  (grasshopper),  391 
Apposition,  192 

Arbacia  (sea  urchin),  78,  160,  330 
Arber,  212 

Arcella  (protozoan),  62,  281 
Archicarp,  223,  290,  312,  313 
Archoplasm,  115,  181 
Arctostaphylos  (angiosperm),  134 
Arisanna  (angiosperm),  104,  105,  248 
Aristotle,  1 
Armadillo,  357 
Arnold,  239 

Artemia  (crustacean),  318 
Artichoke,  134 
Artificial  cytasters,  78,  280 
Artificial  parthenogenesis,  284  ff. 
Ascaris  (nematode  worm) 

centrifuged  egg,  330 

centrosome,  77 

chondriosomes,  118 

chromatin  diminution,  404,  405 

chromosomes  (somatic),  150 

cleavage,  404,  405 

fertilization,  276,  277,  280,  281 

hybrids,  164 

individuality    of    chromosome,    157, 
164 

polar  body,  318 


Ascaris  (nematode  worm)  reduction,  219, 
239,  248,  256,  276 

sex-chromosomes,  358,  359,  367 

spermatozoon,  274 
Ascidian  egg,  329 

Ascobolus  (fungus),  80,  81,  223,  290,  291 
Ascogonium,  312 
Ascomycetes,  centrosomes,  80 

fertilization,  290  ff. 

mitosis,  179 

reduction,  223 
Ascophanus  (fungus),  291,  312 
Ascospore  wall,  80,  291 
Asexual  reproduction,  137 
Askenasy,  193 
Asparagus,  121 
Aspergillus  (fungus),  290,  312 
Aspidium  (fern),  91,  313,  314,  373 
Assimilation,  106 
Aster,  26,  76,   177,   178,   183,   189,  279, 

280,  286 
Asterella  (liverwort),  87 
Astrosphere,  76,  177,  178 
Atamosco  (lily),  315 
Athyrium  (fern),  315,  317 
Atkinson,  223,  248,  290 
Atoms  and  factors,  353 
Atrichum  (moss),  88 
Attraction  sphere,  26,  76,  177 
Auerbach,  16 
Autogamy,  282 
Autosome,  358 
Autumnal  colors,  136 
Avena  (oats),  348 
Axial  filament,  273,  275 

gradient.  139 
Axon,  29,  30 

B 

Bacillus  Butschlii,  67 
Bacteria,  3,  66,  108 
Bacterium  gammari,  67 
Bailey,  63,  177,  192 
Balbiani,  61,  155 
Ballowitz,  274 
Baltzer,  160,  327,  371 
Bancroft,  35,  44 
Banta,  370,  371 
Barber,  38 
Barley,  122 
Barry,  15 
Bartlett,  372 


I  Shi.  \ 


129 


de  Bary,  11,  12,  32,  L08,  312 
Basal  granules  or  corpuscles   96,  97 
Basichromatin,  64,  66,  70,  158 
Basidiomycetes,  centrosomes,  SI 
fertilization,  292 
reduction,  22  I 
Basidiospores,  22  t 
Basidium,  224 
Bataillon,  285,  319 
Bateson,  344,  360,  395 
Bateson  and  Punnett,  378 
Butracoseps  (salamander),  237,  385 
Baur,  332 
Beauverie,  135 
Bechamp,  48 
Bechhold,  37 
Beckwith,  118,  123 
Bee,  318,  319,  357 
Beer,  110,  196,  239 
Begonia  (angiosperm),  137,  256 
Beijerinck,  49 
Belajeff,  86,  90,  95,  288 
Benda,  115,  119 

van  Beneden,  achromatic  figure,  181,  183 
attraction  sphere,  76 
centrosome,  77,  180 
early  work  on  mitosis,  143,  144 
fertilization,  279 

individuality  of  chromosome,  157 
polarity,  138 
reduction,  219 
Bensaude,  292 
Bensley,  47 
Berberis  (barberry),  47 
Berghs,  nucleus  and  mitosis,  61,  66,  158, 
176,  209 
reduction,  231,  236,  257 
Bernard,  69 
Bernhardi,  5 ' 
Beroe  (ctenophore),  328 
Berridge  and  Sanday,  295 
BerthoUetia  (Brazil  nut),  134 
Biochemistry,  23,  414 
Biococci,  69 
Biogen,  42,  50 

Biophore  (Adami),  50,  412$'. 
Biophore  (Weisinann),  49,  226,  400  ff. 
Bioplasts,  33,  48 

Birds,  sex-determination,  363,  367,  369 
Bivalent    chromosomes,    161,    227,    233, 

239 
Black,  292 
Blackman,  M.  W.,  255 


Blackman,  \    II     224,  291,  29  312 

Blackman  and  Fi     er,  290,  312 
Blackman  and  \\  elaford,  291  l.; 

Blakeslee,  290,  35 
Bla  "/    liverwort  .  87,  Q 
Blastomere,  188,  325 
Blending  inheritance,  338 
Blepharoplast,  83  ff. 

relation  to  centrosome,  '»l 
Blochmann,  3 1 s 
Blood  cells,  29,  tin 
Boletus  I  mushroom  .  81 

Bullet,  r.   s7 

Bone,  30 

Bonellia  (gephyrean  worm  .  37 1 

Bonnet .  (  '..  3 
Bonnet.  J.,  223 

Bonnevie    somatic    chromosomes,    150 
L52,  155,  159 

reduction,  250,  251,  257 
Boquet  stage,  247,  248 
Bordered  pit,  191,  192 
Borelli,  2 
Borzi,  46 

Botrychium  (fern  .  236 
Boudiera  (fungus),  '-".»<> 
Bourquin,  lot'..  109 
Boveri,  amitosis,  213 

archoplasm,  115,  l si 

cent rosome,  77.  78,  ls<> 

chromosome  number,  163,  16 l 

experiments    on    echinoderms    etc., 
Hi:;.  164,  325  330 

fertilization,  279,  286 

germ  cells  of  Ascaris,  mi.  i<>:. 

hereidtary  substance,  210 

individuality  of  chromosome,  157 

nuclear  size,  63 

polar  body  in  Ascarti,  31  s 

reduction,  228,  248,  251 

somatic  chromosomes,  l  19 
Bower,  316 
Brachymeiosis,  223 
Brachystola     (grasshoppei  .     158,      160, 

252,  253,  255 
Brauer,  77.  L55,  227,  248,  318 
Braun,  1 1 
Bridges,  382 
Brooks,  !■'.  'I'..  291 
Brooks.  \\  .  K  .    10 
Brown,  H.  B..  ^j\ 
Brown   R.,  ti.  7 
Brown   W.  II..  291,  29 


430 


INDEX 


Brownian  movement,  7 
Bryonia  (angiosperm),  356 
BryophyUum  (angiosperm),  137 
Bryophytes,  blepharoplast,  86 

centrosome,  82 

fertilization,  292,  293 

reduction,  224 

sex-determination,  3.55,  356,  364 
Br  yum  (moss),  317 
Buchtien,  89 
Bud  sport,  351 
Buffon,  48,  398 
Bufo  (toad),  241 
Burrnannia  (angiosperm),  315 
Buscalioni,  211 
Biitschli,  achromatic  figure,  181 

bacteria,  66 

blue-green  algae,  202 

centrosome,  79 

ectoplasm,  42 

furrowing,  188 

polar  bodies,  15 

protoplasm,  33,  49,  183 


C 


Cceoma  (rust  fungus),  312 

Calcium  salts,  135,  194 

Caldwell,  93 

Calkins,  183,  213,  248 

Calopogon  (orchid),  300 

Calycanthus     (angiosperm),      159,     236, 

248,  256 
Calypogeia  (liverwort),  110 
Cambarus  (crayfish),  237 
Cambium,  27,  63,  177 
Camerarius,  13 
Campanula  (angiosperm),  159 
Campbell,  89,  195 
Canaliculse,  48 
Cancer  (crab),  237 
Canis  (dog),  237,  361 
Cannabis  (hemp),  256,  372 
Capsicum  (angiosperm),  332 
Carbohydrates,  41 
Cardiff,  236,  257 
Car  ex  (sedge),  159 
Carloton,  65 
Carnoy,  33,  248 

and  Lebrun,  280 
Carothers,  160,  255 
Carotin,  104,  121 
Carpogonium,  289 


Carrel,  138 

Carruthers,  136,  223,  313 
Carter,  60,  105,  209 
Cartilage,  29,  30 
Carus,  15 
Castle,  390 

and  Wright,  391 
Castrada  (flatworm),  274 
Catalyzer  in  egg,  285 
Cattle,  sex-determination,  361,  371 
Cavers,  119,  122 
Cell,  description,  24 

discovery,  2 

evolution  of,  69 

a  system,  26 
Cell-division,  Chaps.  VIII-X 
Cell-formation,  early  views,  5 
Cell  plate,  176,  190 

sap,  135 

Theory,  7,  9,  12 

wall,  26,  28,  190  ff. 
Cellulose,  135,  194 
Central  spindle,  177,  178 
Centrifuged  eggs,  329,  330 
Centriole,  77,  83 
Centrodesmose,  83,  209 
Centronema,  84 
Centrosome,  26,  76  ff. 

individuality,  77 

in  fertilization,  279 

in  mitosis,  145,  146,  177,  178,  180 

in  plants,  79  ff. 
Centrosphere,  26,  76 
Cephaloiaxus  (conifer),  295 
Ceratiomyxa  (slime  mold),  222 
Ceratozamia  (cycad),  220,  294 
Cerebratulus  (nemertean  worm),  275 
Chaznia  (protozoan),  61,  68 
Chatopterus  (annelid  worm),  275 
Chamberlain,   48,   82,   93,   94,   294,   295, 

298 
Chambers,    observations   on   living   cell, 
38,  43,   62,   77,   118,    119,    120, 
123,  183,  189,  214,  287 
Chara   (stonewort)   and   other   Charales, 
86,  90,  95,   108,   116,  210,  211, 
222,  288 
Chemical  theory  of  development,  412 
of  heredity,  412 

theories  of  life,  50 
Chiasmatype  hypothesis,  257,  384  ff. 
Chick  cells  in  tissue  cultures,  146 
Child,  amitosis,  213 


I  \  DE\ 


Child,  colloids,  3  I 
germ-plasm,  1<>7 
germ-track.   J()."> 
metabolic  gradient,  139 
metabolism  in  maturation,  260 
metaplasm,  133 
nucleoplasmic  ratio,  63 
parthenogenesis,  319 
physiology  of  fertilization,  284 
polarity,  139 
senescence,  136,  137 
vital  activity,  49,  50,  51 
Chitoscyphua  (liverworl  I,  248,  249 
Chimera,  351 

Chironomus  (midge),  61,  405 
Chitin,  194 

Chlamydomonadaceac,  112 
Chloralized    cells,    136,    104,    248,    249, 

259,  260 
Chlorogonium  (green  alga),  85 
Chlorophyll,  104,  105,  124 

inheritance,  331 
Chlorophyllogen,  105 
Chloroplast,  104,  105,  113 

in  heredity,  331-334 
Chmielewskij,  109 
Chodat,  202 
Cholesterin,  41 
Chondriokont,  116 
Chondriomite,  116 

Chondriosomc,  26,  48,  97,   111,   115  jf., 
134 
division,  118 
in  fertilization,  120,  280 
microtechnical  methods,  117 
other  functions,  121 
and  oxidation,  123 
relation  to  plastid,  121  jf.,  124 
rank  as  cell  organ,  124 
role   in   inheritance,    119,    122,    121, 
279 
Chorthippus  (grasshopper),  158,  237 
Christman,  291,  292,  312 
Chromatid,  230,  246,  247 
Chromatin,  25,  40,  64 

contributions  of  parents,  279,  340 
diminution,  404,  405 
extrusion,  136 
qualitatively  varied,  226 
Weismann's  conception  of,  400 
Chromatoider  Nebenkorper,  86,  89 
Chromatophore,  104,  136,  202 
Chromidia,  67,  68,  117,  118,  L33,  208 


<  liKMiinli.il  fragmentation,  Jus 
( Shromidiogamy,  281,  282 

<  Ihromioles,  155 

( Ihromomere  \  esicle,  158,  162 
Chromomeres,    154   jf..    165,   238,   243, 

258,  391,  392 
( Shromosomal  i  esicle,  158 
Chromosomes,  aberrranl  behavior,  302 

alveolation,  148,  I  19 

m  artificial  parthenogei 

bivalent,  227.  233 

in  cleavage  division,  278,  2 

conjugation,  227.  233,  251  jf. 

in  ( lyanophycese,  203,  Jul 

early  work  on.   1  1.; 

in  endosperm,  301 

equational  division,  156,  226 

in     fertilization,     277   279,     295     fj  . 
340 

in   heredity,  326,  330, 
396 

individuality,  1">7   His 

in  life  cycle,  221,  340 

linear  organization,   162,  --*> 

in  man,  361,  362 

map.  388,  389 

in  mitosis,  1  1  \  jf. 

multiple  chromosomes,  164,  165 

and  mutation,  3  1 1  jf. 

number,  162 .//"..  :;  17  jf. 

pairing  before  maturation,  255 

pairs.  160,  161,  227,  251 

in  parthenogenesis,  '■'<  I  8,  319,  36 1 

qualitative  division.  227 

reduction  of,  219  261 

sex,  358  jf. 

size  and  shape,   loll.  J.YJ  jj. 

and  species.  .">  17  if. 

specificity  of,   162,  J-">0 

splitting  of.  |  I:,.  152,  153,  ! 

tetrads,  230,  233,  243  Jf. 

transverse  division,  227 
Chroococcua   (blue-green   alga      68    205, 

206 
Chrysanthemum  (angiosperm  .   -Is 
Chun.  21 1 

Cicada  (inseel  ,  120,  121 
( 'ienkow  Bki,   12,  32 
Cilia.  30.    i:».  96 
Ciliated  cells,  96,  212 
Cirri,  30,  15 

Cladophora  (green  alga  .  60,  85,  105,  112, 
209 


432 


INDEX 


Claussen,  81,  224,  290,  291 
Cleavage  centrosomes  in,  279,  280 
chromosomes  in,  278,  295  ff. 
division,  278,  329 
furrowing,  146,  186  ff. 
Cleland,  109,  223 
Closing  membrane,  191,  192 
Closterium  (desmid),  186,  222 
Clowes,  35,  36,  44 
Clytia  (coelenterate),  328 
Coagulation,  38 
Cobcea  (angiosperm),  177 
Coccidium  (protozoan),  207,  20S 
Ccenocentrum,  289 
Coenocytes,  60,  411 
Cohn,  11 
Coker,  295 

Coleochoste  (green  alga),  113,  222,  236 
Collema  (lichen),  291 
Collins,  E.  J.,  357 
Collins,  G.  N.,  333,  349 
Colloids,  34 

Colonial  alga?,  140,  331,  334,  411 
Colorblindness,  381 
Components  (of  colloids),  35 
Compositae,  236,  315 
Coniferales,  fertilization,  294  f. 
Conjugate,  288,  316 
Conjugate  division  of  nuclei,  292 
Conjugation  in  Paramoecium,  283,  284 
Conklin,  achromatic  figure,  183 
amitosis,  213-214 
centrosomes,  78 
fertilization,  279,  280 
individuality  of  chromosome,  159 
nucleoplasmic  ratio,  63 
performationism,  411,  414 
promorphology  of  ovum,  329,  330 
role  of  chromatin,  70 
Weismannism,  408 
Connecting  fibers,  145,  176 
Connective  tissue,  29,  30 
Conocephalus  (liverwort),  86 
Constrictions  in  chromosomes,  160,  161, 

162,  166,  248,  249,  255,  260 
Continuity  of  chromosomes:  see  chromo- 
somes, individuality 
of  germ-plasm,  402,  403,  413 
Contractile  vacuole,  30,  44,  46,  84 
Contractility  in  achromatic  figure,    183 
Copepods,  245,  249,  405 
Coprinus  (mushroom),  292 
Corallina  (red  alga),  80,  223 


Cork,  2,  194 

Corpuscles  de  rebut,  283,  284 

Corpuscular  theories,  391,  398  jf. 

Correlation,  139 

Correns,  331,  332,  338,  356,  366,  372 

Corsinia  (liverwort),  82,  293 

Corti,  6 

Corydalis  (angiosperm),  211 

Coulter.  288 

and  Land,  295 
Cowdry,  E.  V.  116-118 
Cowdry,  N.  H.,  116,  117,  123 
Crampton,  328 

Crepidula    (mollusk),    amitotic    appear- 
ances, 213 
fertilization,  279,  280 
individuality  of  chromosome,  159 
nuclear  size,  63 
sex-determination,  371 
Crepis  (angiosperm),  159-162 
reduction,  239,  252,  254 
species  and  chromosomes,  349 
Crinoid,  326,  327 
Criss-cross  inheritance,  380 
Crossing  over,  385,  386  f. 
Cruciferee,  159 
Cryptobranchus    (amphibian),    159,    160, 

278,  279 
Cryptomeria  (conifer),  294 
Crystalloid,  35,  65 
Crystals,  134,  135 
Ctenolabrus  (fish),  160 
Ctenophore,  328,  329 
Cuenot,  355 

Culex  (mosquito),  165,  256 
Cumingia  (mollusk),  286 
Cumulative  factors,  343 
Cunninghamia  (conifer),  295 
Cuscuta  (angiosperm),  47 
Cuticle,  194 
Cuticularization,  194 
Cutin,  194 
Cutinization,  194 
Cutleria  (brown  alga),  222,  241 
Cutting,  312 

Cyanophycese,  cell-division,  202  ff. 
nucleus,  68 
plastids,  108 
Cyanophycin  granules,  203,  206 
Cyanoplasts,  203 
Cycas  (cycad),  93,  294 
Cyclops   (copepod),    159,    160,   237,   245, 
279 


INDEX 


I.;.; 


Cystolith,  134,  135 

Cytoblast,  8 

Cytoblastema,  8,  1  i:; 

Cytokinesis,  186  ff, 

by  cell  plate,  176,  190 
by  furrowing,  L86 

Cytolysis,  285,  286 

Cytomorphosis,  1 17 

<  Jytoplasm,  25,  40 

in  fertilization,  280,  294,  299 
in  heredity,  326,  328,  330 
pattern  in  ovum,  329 
reducing  action,  123 

Czapek,  44 


D 


Dahlgren,  225 

Dahlia  (angiosperm,),  2.56 

Dale,  290,  312 

Dangeard,  84,  85,  86,  122,  202,  290, 

Daphnia  (copepod),  371 

Darling,  159,  356 

Darwin,  16,  49,  398,  402 

Datura  (angiosperm),  134 

Davey  and  Gibson,  372 

Davis,  B.  M.,  blepharoplast,  85 

eentrospheres,  80,  82 

cytokinesis,  187 

fertilization,  289,  290 

(Enothera  mutants,  344,  345 

plastid,  113,  114 

reduction,  241 
Davis,  H.  S.,  239 
Death,  136,  138 
Debaisieux,  237 
Dedifferentiation,  137 
Degeneration,  2\\  ff.,  259 
Dehorne,  150,  159 
Delage,  63 

and  Goldsmith,  400 
Delia  Valle,  103 
Dellinger,  45 
Dendrite,  30 
Denke,  195 

Derbesia  (green  alga),  85,  86 
Dermatosome,  193 
Dermocarpa  (blue-green  alga),  201 
Derschau,  66,  123,  136 
Des  Cilleuls,  212 
Desmid,  105,  186,  222 
Determinant.  226,  400  ff. 
Deton,  237 
28 


I  teutoplasm,  L3  l 
Development    (Ontogen. 

15,  I  in 
chemical  <-<»nt  rol  of,  1 1  j 
non-factorial  1 1  1 1 1 

R  i  ismann'fl  I  heory  of,  mi .  107 

Developmental  mechanics,  1 !  1 

Devi-.'.  182 

Dextrin,  107 
Diakinesis,  231,  232,  2 
Dicroccdium  (flatworm  .  245 
Dictyota  (brown  alga  .  cell  wall,  l 

centrosome,  79,  so.  82 

reduction,  223 
Didiploid  nuclei,  260 
Didymium  (slime  mold  .  1^7 
Dietel,  22  I 

Differentiation  of  cells,  26,  137 

products.   133  Jf. 
Digametic  condition.  :;.">s  /f. 
291        Digby,  extruded  chromatin,  L36 

nucleolus,  65 

prochromosomes,  L59 

reduction,  239,  240,  248 

somatic  chromosomes,  1  19,  152 
DinophUtts  (annelid  worm),  :;.".7.  368 
Dionoea  (angiosperm  .  17 
Dioon  (cycad),  02.  '.»:;.  94,  294 
Diplotene,  231,  233 
Diptera,  256 
Dispermy,  163,  326 
LH8808teira  (grasshopper),  119 
Dixon,  211 
Dobell,  67 
Dodge,  22:; 
Dog,  361 

I  >ominance,  .;.'i7.  339 
Doncaster,  i\.  319,  363 
Double  fertilization.  14,  298,  299,  302 
Double  heterotypic  spireme,  241   243 
Double  reduction,  223,  224,  291 
Douin,  •">•">•">,  36  l 
Downing,  275 

Draparnaldia  (green  alga  .  104,  108 
Driesch,  328,  U2 

and  Morgan.  328 
Drosera   (angiosperm    159,  236,  251,  252 
Drosophila  (fruit  fly),  chromosome  com- 
plement, 350,  383 
map,  388,  389 

crossing  over.  3^7  jf. 

intersexes,  37 1 .  376 

linkage,  "'7'.i 


434 


INDEX 


Drosophila    (fruit   fly),   non-disjunction, 
382,  383 

sex-chromosomes,  360  ff. 

sex-linkage,  380 
Drosophilidse,  349,  350 
Druery,  311 
Druner,  181,  183 
Druse,  134,  135 
Dublin,  255 
Dubreuil,  121 
Duesberg,  115-120,  361 
Dujardin,  11 
Dupler,  295 

Duplex  chromosome  group,  340 
Duplicate  factors,  343 
Dytiscus  (beetle),  237 

E 

Eames,  295 

and  Hayes,  135,  333 
East  and  Jones,  352 

Echinoderm  hybrids,   160,   163,  325-327 
Echinus  (sea  urchin),  160,  325 
Ectocarpus  (brown  alga),  222,  314 
Ectoderm,  329 
Ectoplasm,  42,  44,  45 
Ectoplast,  25,  43,  44,  85 
Egg,  segmentation  in  animal,  188,  189 

fragments,  287 
Eisen,  66,  155 
Elasis  (palm),  134 
Elaioplast,  109,  110 
Elasmobranchs,  239 
Elastin,  194 

Electrical      charge      of      nucleus      and 
cytoplasm,  62,  184 

phenomena  in  egg,  287 

theories  of  mitosis,  184 
Elementalism,  12,  411 
Elodea  (angiosperm),  37,  122,  356 
Emberger,  122 
Embryo  sac,  299 
Embryogeny      (see     Development) 

historical,  13,  15 
Embryonal  mitosis,  278,  295  ff. 
Embryonic  cells,  137 
Emerson,  135,  302,  333,  384,  391 
Emulsion,  34-36,  44 
Emulsoid,  35 
Enchenopa  (bug),  237 
Enchylema,  33,  34 
End  piece,  275 
Endoderm,  329 


Endoplasm,  42,  44 

Endosperm,  development  of,  302 

mosaic  effects,  302 

nuclear  behavior  in,  300-301 
Endospore,  195,  196 
Energy  change  in  cell,  26,  52,  70 
Entelechy,  412 
Enteroxenos  (mollusk),  237 
Entz,  83,  84 
Enucleated  cells,  69,  70 
Environment  and  development,  411,  413 
Enzyme,  39 

Ephedra  (gymnosperm),  295 
Epigamic  sex-determination,  357 
Epigenesis  theory,  4 
Epithelial  cells,  30,  83,  96 
Equational  division   of  chromatin,    156, 

226,  228,  401 
Equisetum    (pteridophyte),    chromosome 
tetrad,  248 

dicecism,  356 

spermatogenesis,  90-92,  95 

spore  coat,  196 

stem,  135 
Ergatule,  49 
Erhard,  96 
von  Erlangen,  189 
Ernst,  300,  301,  315 
Erysiphe  (fungus),  80,  81,  290 
Escoyez,  79,  87,  210 
Etiolation,  105 
Euchlcena  (angiosperm),  349 
Eudorina  (green  alga),  112 
Euglena  (flagellate,  eyespot,  112 

flagellum,  45 

mitosis,  209 

paramylum,  108 
Euschistus  (bug),  237 
Exine,  196 
Exospore,  195,  196 
Extruded  chromatin,  136 
Eye  color  in  Drosophila,  380 
Eyespot,  46,  84,  111  #.,  112 


Factor   (see  Gene),  333,   334,   336,  353, 

409,  410 
Factorial  hypothesis,  343,  410 
Farlow,  311 
Farmer,  220,  248 

and  Digby,  136,  239,  312-315 

and  Moore,  237,  239 


I.\  DEX 


Farmer  and  Reeves,  82 
and  Shove,  239 

and  Willi:. ins,  so,  222 
Fair,  186-188 
Fasciola  (flatwonn),  'J  17 
Fasten,  237,  257 
Fats,  41,  134 

and  chondriosomes,  121,  122 
I  a i ill,  80,  81,  179,  223,  291 
Faurc-Fremiet,  115,  117,  lis 
Fedorley,  252 

Fegatella  (liverwort),  86,  87 
Ferguson,  295 
Fertilization,  animals,  273  jj. 

centrosome  in,  279 

cone,  277 

historical,  13 

membrane,  277,  284 

parental  chromosome  groups  in, 
159,  254,  276,  278,  279,  295, 
300,  340 

physiology  of,  284  ff. 

plants,  287  ff. 

relation  to  maturation  of  ovum,  275 
Fertilzin,  287 
Fiber  theory,  6 

Fibrillar  theory  of  protoplasm,  33 
Fick,  163,  249,  250 
Ficus  (rubber  plant),  134,  135 
Filaria  (roundworm),  120 
Fischer,  34,  66,  108,  202,  204 
Fish  hybrids,  159,  160 
Fitting,  194,  195 
Fitzpatrick,  224,  291 
Fixation,  38,  117 

Flagellates,  blepharoplast,  83,  111 
Flagellum,  45,  84,  96,  112 

of  spermatozoon,  273-275 
Flemming,  amitosis,  211,  213 

cell-division,  143 

centrosome,  77 

chondriosomes,  115 

chromatin,  64 

chromosomes  in  man,  361 

nuclei  by  division  only,  10 

protoplasm,  33,  34 

reduction,  220,  227 

spindle,  ISO 
Florideae,  108,  116,  223 
Floridean  starch,  108,  109 
Florin,  82,  248,  249 
Flower  colors,  135 
Focke,  302 


/■  /  niculum  (angiosperm  .  134 
Fol,  aster  and  asl  roephen  .  76 

Cell-division,   1  |.; 

cent  rosomes  in  fertilizatioi 

chromomeres,  155 

fertilization,  l"> 

-i  reaming  of  protoplasm,  1 
Fontana,  6 
I 'ood  materials,  l 
Foot  and  Strobell,  213,  244,  245 
Forenbacher,  lis,  122 
Forficula  ^earwig  .  -'•!'.» 
Fo88ombronia  (liverworl  ,  248 
Fowl,  361,  363 
Krauze,  112 
Fraser,  brachymeiosis,  223 

centrosome,  si 

fusion  in  ascomycetes,  290,  312,   '>\  '- 

reduction.  238,  239,  240 

somatic  chromosomes,  l  19 
Fraser  and  Brooks,  si,  223 

and  Snell,  149 

and  Welsford,  81,  223 
Free,  44 
Fremy,  193 
Fries,  224 
Friesendahl,  236 
FrUiUaria  (angiosperm  .  amitosis,  211 

constricted  chromosomes,  162 

fertilization.  298,  300,    '.<>1 
Frog,  artificial  parthenogenesis,  319 

blastomeres,  328 

centrifuged  eggs,  :;:;,) 

cleavage,  188 

organization  of  egg,  330 

sex-determination,  368,  370 
Fromman,  33 
Fuchsia  (angiosperm  l,  21 
Funis,  centrosome,  s".  B2 

rv.-pot,    111 

fertilization,  l  1 

gametes,  288 

parthenogenesis,  31 1 

plasma  membrane,  43 

reduction,  222.  223,  24] 

spindle,  179 
Fujii,  92 

Fuligo  (slims  mold  .  39,  1 1 .  1  s~ 
Fuim a  |  moth 

Fund rin     mOSfl   .  106,  ■'>■'<> 

Function  and  si  ructure,  37  I 
Fumlulus  (fish  .  158,  159,  160,  328 
Funkia  (angiosperm  .  236,  266 


436 


INDEX 


Furrowing  (cytokinesis),  186  jf. 
Fusion,  centrosomes,  280/291 

chromosomes,  257,  394 

gametes,  276  ff. 

nuclei,  15,  219,  211  ff.,  299,  301 

sporocytes,  313,  314 

three  nuclei  in  endosperm,  299,  301 


G 


Gaertner,  13 

Gaillardia  (angiosperm),  110 
Gall-fly,  319,  364 
Gallactinia  (fungus),  80 
Gallardo,  184 
Galton,  399,  402,  404 
Galtonia      (angiosperm),     extrusion      of 
chromatin,  136 

nucleolus,  65 

reduction,  236,  239 

somatic  pairing,  256 
Gametes,  137,  273  ff.,  288,  293  ff.,  333  ff. 
Gametophytes,      sex-determination      in, 

355,  356,  364,  372 
Garber,  292 
Gardiner,  46,  47 

and  Hill,  46 
Gardner,  108,  202,  204 
Gargeanne,  110,  111 
Gastrodia  (orchid),  314 
Gatenby,  120 
Gates,  63,  149,  241,  252,  344,  345 

and  Thomas,  345 
Gaudissart,  121 

Geddes  and  Thomson,  355,  369 
Geerts,  241,  252 
Gegenbaur,  15 
Gel,  35 

Gelatin,  37,  194 
Gelation,  38 
Gemini,  233 
Gemmule,  49,  226,  398 
Generative  apogamy,  315 
Generatule,  49 
Genes,  343 

cytological  evidence  for,  391  ff. 

in  development,  410 

linked,  380,  386 

modification  of,  351-353 

nature  of,  352,  391,  409 
Genetic  continuity,  15,  323 
Genotype,  339 
Geotriton  (salamander),  118 


Gerassimow,  61,  69 

Germ  cells  and  soma  cells,  402  ff. 

Germ-plasm,  226,  334 

and  soma-plasm,  407 

Weismann's  conception  of,  400  ff. 
Germ-track,  403  ff. 
Germinal  localization  theory,  328 

selection,  404 

vesicle,  62,  221,  275 
Giard,  15 
Gigantism,  346 
Giglio-Tos,  250 
Gille,  160 

Ginkgo  (gymnosperm),  92,  122,  236,  294 
Gladiolus  (angiosperm),  177 
Glaser,  40,  213 
Glceocapsa    (blue-green    alga),    204,    205, 

206 
Globular  theory,  6 
Globulin,  40 
Glucose,  106 
Glucoside,  135 
Glycogen,  108 
Gnetales,  294 
Gnomonia  (fungus),  291 
Godlewski,  119,  213 

echinoderm  hybrids,  326-328 
Goldschmidt,  chromidia,  133 

parthenogenesis,  319 

reduction,  244,  245,  256 

sex-determination,  370 
Gonium  (green  alga),  112 
Gonotokont,  225 
Goodsir,  10 
Goroschankin,  46 
Gottsche,  110 
Gould,  371 

Grafted  tissue  in  apogamy,  312-314 
Graham,  82,  293 
Granata,  250 

Granular  theory  of  protoplasm,  33,  49 
Grasshopper,    156,    158,    162,    233,    237, 

243,  258,  351,  391,  392 
Gregoire,  chromomeres,  258 

nuclear  reticulum,  64,  150,  152,  155 

reduction,  227,  231,  234,  236,  245, 
248,  257,  259 
Gregoire  and  Berghs,  82 

and  Wygaerts,  64,  149,  152,  155 
Gregory,  239,  346,  391 
Grew,  2 

Griffithsia  (red  alga),  223 
Griggs,  239,  248 


/  \  DEX 


137 


Gromia  (protozoan),  45 

(1  round  substance,  ll'A 

Growth  period  in  oocyte.   158,  221,  233, 

234,  274,  275 
Gruber,  61,  69,  To 
Gryllotalpa  (mole  cricket),  118,  119 
Guignard,  blepharoplast,  '.*<),  94 

eyespot,  111 

fertilization,  14,  298,  300,  325 

reduction.  220 
Guilliermond,  bacteria,  67 

eentrosome,  NO 

chondriosomes,  115  122 

reduction,  224 

yeasts,  211,  292 
Guinea  pigs,  274,  341,  342 
Gurwitsch,  213 
Gutherz,  361 
Gutta  percha,  135 
Guyer,  285,  361,  363 
Gyrrmogramme  (fern),  90 
Gyrodactylus  (flat worm),  160 


II 


Baberlandt,  46,  61,   103,   104,   105,   193 
Haeckel,  16,  48,  324,  402 
Haecker,  fertilization,  279 

hereditary  substance,  210 

individuality    of    chromosome,    lf>7. 
159 

Keimbahn-plasma,  405 

nucleolus,  66 

pseudoa mitosis,  211,  213 

reduction,  227,  244,  245 

sex-determination,  357 
Hamiatochrom,  112 
Haller,  6 
Hamm,  14 
Hammarsten,  40 
Hance,  165,  346 
Hannig,  196 

Hanstein,  12,  24,  33,  46,  133 
Hardy,  62 
Bargitt,  213 
Barman,  160,  213,  391 
Harper,  centrosomes  in  a8CU8,  NO.  81 

chondriosomes,  123 

cleavage  furrow,  187 

development  in  colonial  algse,  334, 
411 

evolution  of  cell  structure.  207 

fertilization  in  ascomycetes,  290 


Harper,  inheritance  in  colonial  alga?,  • 
334 
mitosis  in  myxon  208 

plastids,  103,  105,  L07,  lb". 
polarity,  138,  l  10 
protoplasm,  38,  19,  ">i 
reduction  in  ascomyceti 

Hart  niann  and   Noller,  s  \ 

Bartog,  KM.  185 

Harvey.   E.   E.,   162 

Barvey,  E,  V.  284 

Harvey,  Win..    1 
Hasper,    105 
Batschek,  36,  19,  138 
Bautschicht,  12 

Heart  wood,   194 

Hegler,  108,  202,  203 

Begner,  62,  104,  105 

Beidenhain,  64,  70,  138,  III.  18 

Beilbrunn,  189,  286,  287 

Helix  (snail  .  96,  256 

HeUeboru8  (angiosperm  .  159 

Helvetia  (fungus  .  L36,  223,  291,  313 

Bemiptera,  239,  255 

Hemp,  372 

Benking,  227.  244,  251,  256,  358 

Henle.  10 

Benneguy,  7o.  95,  96 

Berbst,  213,  325,  326 

Heredity,  cell  organs  in,  323  ff. 

chemical  theory  of,    112 
in  colonial  algS,  331 .  33  1 
early  theories  of,  ">-,s  Jf. 
Mendelian.  336  ff. 
non-factorial  theories  of,   1 1  1 
Weisniann's  theory  of,    101 

Berla,  157.  164 

Berlant,  285,  286 

Hermann,  1 1 1.  180,  183 

Berrick,  27  l 

//.  rsiliti  (copepod  .  237 

I  bit  win,  < )-.  basal  granul< 
cell-division,  1  13 
fertilisation,  15,  273,  276 

nucleus  in  heredity,    l'».  32  I 

spindle,  ls<> 
Bertwig,  R.,  amitosia,  211,  213 
chromidia,  133 
division  in  protOSOa,  207 
heterotypic     prophase     an     abort 

mitosis,  259 
Ducleoplasmk  rat i<>.  63,  70 
Bex-determination,  368,  37(  I 


438 


INDEX 


Hesperotettix  (grasshopper),  164,  165 
Heterochromosomes,  358  ff. 
Heterodera  (fungus),  260 
Heterogametic  females,  363 

males,  358  ff. 
Heterotypic    mitosis,   220,   231  ff. 

compared  with  somatic  mitosis,  228 

and  degeneration,  259-260 
Heterozygous  state,  339 
Heuser,  144 
Hexads,  249 
Hick,  46 

Hieracium  (angiosperm),  315,  316 
Hill,  46 
Hilum,  106 
Hirase,  92,  294 
His,  328 
Hober,  51 

Hofmeister,  10,  13,  14 
Holt,  165 

Homarus  (lobster),  274 
Homoeotypic  mitosis,  220,  231  ff. 
Homologous    chromosome    pairing,  161, 

252-254,  340,  342 
Homozygous  state,  339 
Homunculus,  3 
Hooke,  2,  23 
Hoppe-Seyler,  40 
Hormones,  139 
Horse,  361 
Hoven,  121 
Humaria    (fungus),    81,    223,    290,    291, 

312,  313 
Hi'ippe,  67 
Hutchinson,  295  ff. 
Huxley,  12,  13,  168,  273 
Hyaloplasm,  33,  42,  183 

sphere,  76 
Hybrid  echinoderms,  160,  163 

fishes,  159,  160 
Hydatina  (rotifer),  357,  364,  368 
Hydnobolites  (fungus),  81,  224 
Hydra  (coelenterate),  275 
Hydrocharis  (angiosperm),  256 
Hydrodiclyon  (green  alga),  85,  105,  107, 
109,  187 
development  of,  334,  411 
inheritance  in,  331,  334 
Hydrogel,  37 
Hydrophilus  (beetle),  239 
Hydrosol,  37 

Hygrophorus  (fungus),  224 
Hymenomycetes,  224,  292 


Hypertonic  solutions,  285 
Hypothesis,  role  of,  396-397 


Id,  226,  400  ff. 
Idant,  226,  401 
Idiochro matin,  208,  209 
Idiochromosome,  358 
Idioplasm,  258,  324,  400 

chemical  conception  of,  412 
theory  of  Nageli,  399 
Idiosome,  77 
Ikeda,  248 

Ikeno,  86,  89,  93,  94,  294,  332 
llyanassa  (mollusk),  328 
Immortality  of  certain  cells,  138,  403 
Independence  of  chromosome  pairs,  255 
Individuality,  aleurone  grain,  135 
alga  colony,  140 
centrosome,  77 
chromosome,  157  ff.,  168 
organism,  168 
plastid,  113 
Infusoria,  60 

Inheritance  of  acquired  somatic  changes, 
399,  403 
colony  characters,  331,  334 
Mendelian  characters,  336  ff. 
plastid  characters,  331  ff. 
sex,  354  ff. 
Insects,  160,  239,  359  ff. 
Inter-alveolar  substance,  34 
Interference,  389,  390 
Inter-filar  substance,  33 
Interkinesis,  232,  236 
Interphase,  145,  150,  151,  236 
Intersexes,  370  ff. 
Intracellular  pangenesis,  399 
Intranuclear  spindle,  179 
Intra-vitam  stains,  117,  287 
Intussusception,  193 
Inulin,  134,  135 
Inversion  of  phases,  36,  44 
Ipomcea  (angiosperm),  196 
Iris  (flag),  121,  122,  177 
Ishikawa,  256 
Isoetes  (pteridophyte),  114 
Isonandra  (angiosperm),  135 
Isotropy  of  egg,  328 


Jaeger,  402 

Jahn,  59,  208,  222 


INDEX 


139 


Janssen,  1 

Janssens,  237,  257,  385,  386,  393 

and  Willems,  237,  256 
Jennings,  »'»:;.  396 
Johnson,  1 1 1 
Johow,  210 
Jonsson,  li> 
Jordan,  213 
Jorgensen,  178,  213 
Juel,  224,  315 

Juglans  (walnut,  butternut),  L34 
.1  iini pi  rns    juniper  i.  294,  2,.».") 


K 


Kabsch,  193 

Kahle,  404 

Karsten,  79,  209,  222 

Karyochondria,  118 

Karyogamy,  282 

Karyokinesis,  143  (sec  Mitosis) 

Karyolvmph.  25,  64,  149,  182,  184 

Karyoplasm,  25 

Karyoplast,  84 

Karyosome,  25,  65,  151,  209 

Kassowitz,  133 

Keene,  110,  290 

Keimbahn,  403 

-determinants,  lo<> 

-plasma,  406 
Kemp,  259 
Keratin,  19  1 
Kernplasma  relation,  62 
Keuten,  209 
Kienitz-Gerloff,  4<>,  47 

Kieser,  5 

Kihara,  252,  348 
Kildahl,  295 
Kinetonucleus,  8 1 
Kinetosomes,  ss 
King,  241,  368 
Kingery,  116,  259 
Kingsbury,  117,  L23,  124 

ami  Hirsch,  259 
Kinoplasm,  '■>  1.  t3,  l sl 
Kinoplasmic  caps,  1 75 
Kite,  38,  13,  62,  64 
Kiel,.,  in.  L12,  L93 
Klein.  :;:;.  181,  183 
Kniep,  22  1 
Knoche,  213 
Koelreuter,  13 


Kofoid,  208,  209 
Kohl,  Mi.  His.  202,  203 

von  Kolliker,  lo,  l.",.  16,  :;i 

Koltzoff,  274 

Konopacki,  213 

Korff,  it.").  96 

Kornhauser,  237,  257 

Korotneff,  1  In 

Korechelt,  61,  24  1 

Kossel,  in.  12 

Kostanecki  and  Wierzyski,  276 

Kowalevsky,  I  1 1 

Kuczynski,  8 1 

Kuhla,  1»>,  17 

Knline,  12 

Kiirss.-inou  ,   1  ).;.  222 

Kuschakewitsch,  368 
Kusano,  314 
Kuster,  101,  110 
Kuwada,  <»">,  256,  ■'>  Is.  '■'<  19 
Kylin,  222.  ill  1 


Laboulbenia  (fungus),  179,  22  1.  291 
Lachnea  (fungus),  80,  81,  223,  290,  291, 

312 
Lagerburg,  236 
Laibach,  159 
Land.  294 
Lang,  213 

Larix  (larch  .  122.  L82,  L91,  192,  2 
Lastrcea  (fern),  312,  313,  314,  317 
Laticiferous  vessel.  59 
Lauterborn,  7'.' 

La  Valette  St.  George,  1  1.  1 15,  IS 
Lawson,  fertilization.  294,  295 

nuclear  membrane   6  1 

reduction,  239 

spindle.   177.   1M 
Lecithin,  11 
van  Leeuwenhoek,  3 
win  Leeuwen-Reijnvaan,  221 
Leguminose,  •'>  Is 
Lenhossdk,  96 
Leolia  ^fungus),  291 
Lepeschkin,  1 1    19 
Li  pidoi    ■       li-li  .  162 
Leptonema,  231  233 
Leptotene,  23 1 
Lerat,  237,  245 

Leucoplast,  104,  L06,  109,  113,  121,  L22 
Levine,  81,  22  1 


440 


INDEX 


Lewis,  C.  E.,  87 

Lewis,  I.  F.,  223 

Lewis,  I.  M.,  239 

Lewis,  M.  and  W.,  116,  118,  123,  146 

Lewitski,  115,  118,  121 

Leydig,  33 

Libocedrus  (conifer),  294 

Lignin,  194 

LiUum  (lily),  amitosis,  211 

chondriosomes,  116 

extruded  chromatin,  136 

fertilization,  298-301,  325 

megaspore  nuclei,  225 

protoplasmic  connections,  47 

reduction,  236,  238,  239 
Lillie,  F.  R.,  fertilization,  273  ff.,  286 

regeneration,  69 

sex-determination,  370,  371 
Lillie,  R.  S.,  62,  70,  184,  185,  285 
Limosphere,  88,  89 
Lindstrom,  332 
Linin,  25,  64,  180,  181 
Link,  5 
Linkage,  378  ff. 

calculation  of,  389 

groups,  382  ff. 
Lipoid,  41,  43,  44 
Liposomes,  123 
de  Litardiere,  236 
Liverworts,  blepharoplast,  86 

centrosome,  82 

fertilization,  293 

sex-chromosomes,  364,  365,  372 
Lloyd,  44 
Lock,  378 
Locy,  323 

Loeb,  J.,  284,  319,  326 
Loeb,  L.,  138 
Loeb  and  Bancroft,  285 

and  Wasteneys,  70,  287 
Lopezia  (angiosperm),  248 
Lotsy,  225,  344 
Lowschin,  117,  118 
Lundegardh,  123,  144,  149,  155,  236 
Luther,  274 
Lutman,  186 
Lutz,  345,  346 

Lychnis  (angiosperm),  256,  382 
Lycopodium  (pteridophyte),  91 
Lygoeus  ^bug),  359,  360,  367 
Lymantria  ^moth),  370 
Lyon,  194,  195 
Lysin,  285 


M 


Macallum,  66 

Macdougal,  41 

Macfarlane,  47 

Macormick,  290 

Macronucleus,  30,  60 

Macrosomes,  38 

Maggi,  48,  115 

Magnolia  (angiosperm),  188 

Maier,  96 

Maire,  80,  224 

Maize,  chlorophyll  inheritance,  332 

crossing  over,  391 

hybrid  nature?     349 

linkage  groups,  384 
Makinoa  (liverwort),  87 
Male  cells  and  nuclei,  294,  298,  299 

cytoplasm  in  fertilization,  294 
Malone,  237,  361 
Malpighi,  2 
Malsen,  357,  368 
Malva  (angiosperm),  194 
Man,  centrosome,  77 

colorblindness,  381 

reduction,  237 

sex-chromosomes,  361,  362 
Mangin,  194 

Manifold  effects  of  factor,  343 
Mantle  fibers,  145,  176,  177 
Marchal,  317,  355 

Marchantia  (liverwort),  blepharoplast, 
centrosome,  and  spermatogene- 
sis, 82,  86  ff. 

cell-formation,  5 

chondriosomes,  122 

sex-determination,  355 
Marcus,  248,  259 

Marechal,  155,  158,  210,  234,  237,  257 
Mark,  15 

Marsilia  (water  fern),  apogamy,  315 
316 

blepharoplast    and    centrosome,    91, 
92,  95 

nucleus,  61,  66,  210 

spindle,  176 

spore  coat,  194 
Martin,  33 

Martins  Mano,  155,  236 
Masdevallia  (Orchid),  47 
Massee,  46 
Mast,  112 
Mastigella  (flagellate),  83 


INDEX 


141 


Mastigina  (flagellate  .  s;i 

Mathews,  42,  70 

Maturation  divisions,  220  ff. 

Maxiiimw  ,  213 

Mayer,  Rathery,  and  Schaeffer,  123 

McAllister,   109,  236 

McAvoy,  239 

McClendon,  iss,  285 

McClung,  chromosomes  in  insects,  351 

chromosome      individuality,       L60, 
163  -168 

reduction,  245,  252,  393 

sex-chromosome,  358 
McCubbin,  291 
McLean,  212 
Mead,  78 
Mechanism  of  cytokinesis,  L88 

of  karyokinesis,  182 
Mechanistic  hypothesis,  52 
Meek,  185 
Meganucleus,  30,  60 
Megaspore,  Lilium,  225 

Marsilia,  194 

Physostegia,  225 

Selaginella,  195 
Meiosis,  220 

Melandrium  (angiosperm),  256 
Melanoxanthus  (plant  louse),  319 
Melin,  248 
Membranellse,  45 
Mencl,  67 
Mendelism,  336-344 

and  sex,  366 
Mi  nidia  (fish),  160 

Mercurialis  (angiosperm),  250,  356,  372 
Meri.smopcflia  (green  alga),  205,  206 
Meristem,  27,  63,  137 
M<  nutria  (grasshopper),  165 
Merogony,  63,  325 
Merriman,  149,  209,  210 
Mesoderm,  329 
Mesospore,  L95 
Metabolic  gradient ,  139 

rate  or  level,  137,  L39,  260 

theories  of  sex,  369  ff. 
Metabolism,  69,  70.  260 
Metachromatic  205,  206 
Metachrome,  122 
Metaphase,  144,  145,  1  17,  1  18 
Metaplasm,  26,  133  jf. 

.-Hid  Benescence,  136 
Metasyndese,  228 
Metcalf.  208 


Met/.  256,  349,  l.  395 

Meves,  amitosis,  213 

cent  rosome  in  Bperm, 

chondriosomes,  1 15   1 2 1 
in  fertilization,  120,  280 

reduction,  227.  250 

spermatozodn,  274 

spindle,  178 
Meyen,  6 

Meyer,  A.,  if..  106,  L07,  L13,  u  I 
Meyer,  K  .  82,  2 
Miastor  [fly  .  MM    106 
Micella*,  L07,  L93,  399 
Microcycas  (cycad  .  93 
Microdissection,  38,  62 
Micromeric  theories  of  protoplasm,  iN 
Micronucleus,  30,  60 
Microscope,  1 
Microsome.  33,  122,  190 
Microsphoera  [fungus  .  M 
Microsporocvte,  cytokinesis,  187 

reduction.  225.  235,  238 
Microzyme,  18 
Mid-body,  178,  179,  188 
Middle  lamella.   190,   191,   192,    194 

piece,  273,  275,  281 
Miescher,  40,  41 
Migula,  67 

Mimosa  [angiosperm  I,  17 
Minchin,  14,  45.  60,  68,  83,  207,  208,  222, 

281,  282,  317 
Minot,  63 

MirdbMs     (angiosperm),    plastid    inheri- 
tance, 331 

Mendelian  behavior,  338,  339 

spore  coat,  196 
Mirande,  116 
Mirbel,  5 

Mitochondria,  115  ff. 
Mitokinetism,  ls5 
Mitome  and  paramitome,  33 
Mitoplast,  122 
Mitosis,  duration  of  phases,  l  M'> 

heterotypic.  220,  231  ff. 

mechanism  of,   L82  jf. 

somatic,  1  h">  jf. 
compared  with  heterotypic,  228 
summarised,  156 

in  ( Jyanophyces,  202  ff. 

in  other  thallophytes,  1 79 

m  protozoa,  207  ff. 
Mixochromosome,  257 
Miyake,  87,  92,  236,  290   295 


442 


INDEX 


Mnium  (moss),  87,  88,  317 

Moenkhaus,  160 

von  Mohl,  cell-formation,  6.  9,  10 

cell  wall,  190,  192 

plastid,  12,  103 
Moira  (echinoderm),  160 
Moldenhavver,  5 
Molisch,  193 
Monascus  (fungus),  291 
Monaster,  286 
Monkey,  eentrosomes,  77 
Monotropa  (angiosperm),  70 
Monteverde  and  Lubimenko,  105 
Montgomery,  65,  163,  228,     237,     239, 

251,  256,  257,  281,  358,  361 
Moore,  A.  C,  248 
Moore,  B.,  43 

Moore,  J.  E.  S.,  95,  227,  255 
Moore  and  Embleton,  239 
Moreau,  118,  122,  187 
Morgan,  achromatic  figure,  183 

blastomeres,  328 

centrifuged  eggs,  330 

conjugation  in  Paramoecium,  283 

crossing  over,  379  ff. 

cytasters,  78,  286 

factorial  hypothesis,  344,  410 

linkage,  379  ff. 

sex-determination,    357,    360,    361, 
368,  376 

on  Weismann,  408 
Moroff,  213 
Morphoplasm,  400 
Morris,  160 

Morus  (mulberry),  256,  372 
Mosaic  endosperm,  302 

leaves,  331  ff. 
Mosses,  regeneration,  317 

sex-determination,  355  ff. 

spermatogenesis,  88  ff. 
Moths,  sex-determination,  363,  367 
Motile  cells,  83 
Mottier,  blepharoplast,  86,  87 

cell  plate,  186 

eentrosomes,  79,  80 

chondriosomes,  119  jf. 

chromomeres,  155,  238 

fertilization,  298,  300 

reduction,  238,  239 
Mucor  (mold  fungus),  62 
Muller,  389,  390 

and  Altenburg,  352 
Muller,  149,  152,  155,  156,  253,  256 


Mulsow,  359 

Multiple  chromosomes,  164,  165 

complex,  165 
Multipolar  mitosis,  163,  325,  326 

stage  of  mitosis,  176,  177 
Murbeck,  313,  315 
Murrill,  295 
Mus  (mouse),  237 
Musa  (banana),  159    . 
Musca  (fly),  360 
Muscle  cells,  28,  29,  120-121 
Mutation,  344-352 

and  chromosome  number,  344,  351 

of  gene,  351-353 

rate  of,  352 

vegetative,  351 
Myoneme,  45 
Myrica  (angiosperm),  372 
Myricaria  (angiosperm),  236 
Myristica  (angiosperm),  134 
My  .vine  (fish),  237 
Myxomycetes,  chondriosomes,  116 

cytokinesis,  187 

mitosis,  208,  209 

nucleus,  59 

protoplasm,  32 

reduction,  222 


N 


Nabours,  391 

Nachtsheim,  318,  357 

Nagai,  373 

von  Nageli,  cell-formation  by  division,  9 

cell  wall,  190,  192 

idioplasm  theory,  399 

plastid  and  starch,  12 

starch  grain,  107 
Nagler,  282 

Arajas  (angiosperm),  160,  161,  253 
Nakahara,  66,  211,  239 
Nakanishi,  67 

Narcissus  (angiosperm),  346 
Narcotics,  259,  260 
Nathansohn,  211,  213,  316 
Nawaschin,  M.,  160,  162 
Nawaschin,  S.,   14,   162,   298,  300,  301, 

325 
Nebenkern,  91,  120 
Nemalion  (red  alga),  controsomes,  80 

pyrenoid,  108,  109 

reduction,  223 
Nematus  (saw-fly),  318 


INDEX 


1 1 3 


Nernee,  149,  152,  213,  259 
NeoHetta  (fungus),  81,  224 
Nephrodium  (fern),  apogamy,  313  315 

fertilization,  293 

reduction,  232,  236 

spermatogenesis,  91 

spindle,  176 
Nereis  (annelid  worm),  274  277,  280 
Nerve  cells,  29,  30 
Neuroterus  (gall-fly),  319,  Mill 
Newman  and  Patterson,  357 
Newport,  L5 
Nichols,  255 
Nichols,  G.  E.,  295 
Nicotiana  (tobacco),  187 
Nidularia  (fungus),  224 
Nienburg,  291 
Nissl  substance,  30 
Noll,  193,  355 

Xon-disjunction,  346,  348,  382,  383 
Non-factorial    theories   of  heredity  and 

development,  411 
Noren,  294,  295 
Nothnagel,  achromatic  figure,  182 

fertilization,  298-301 

reduction,  239,  240 
Nowikoff,  213 
Nuclear  membrane,  63,  149,  154 

migration,  312 
Nucleic  acid,  40 
Nuclein,  40,  64 
Nucleolini,  65 
Nucleolo-centrosome,  209 
Nucleolus,    25,    65,   150,   154,   181 

and  chromosomes,  209,  210 
Nucleoplasm,  25 
Nucleoplasmic  ratio,  62 
Nucleo-proteins,  40 
Nucleus,  division,  143^". 

discovery,  7 

distributed  nucleus,  59,  68 

early  views  on  division,  10,  143 

evolution  of,  206 

in  fertilization,  277 

functions,  69 

in  heredity,  16,  324  ff. 

kinetic  nucleus,  84 

nuclear  membrane,  ti.'!.   1  19,  154 

in  protista,  tit; 

relation  to  metabolism,  *'»'.• 

sphere  of  influence,  62 

si  ructure,  63,  150 

trophic  nucleus,  s  l 


Nucleus^  vesicular,  69 
Numerical  reduction,  229   2  19 
Nussbaum,  t02 
\  ympkcea    water  lilj   .  l  15,  116 

t  > 

Octads,  249 
(Edamatin,  6  1 

(Edogonium  (green  alga  .  blepharopla 
85 

fertilization,  1  1,  288 

reduction,  222 
(Elkers.  222 
GEnothera,  cytology  of  mutants,    ill  jF. 

fragmentation  of  chromosomes,  I 

synapsis  in  hybrids,  252 
Ogata,  65 
Oil,  110,  111,  122,  134,  I'M 

body,  110,  134 

droplet,  division  of.  189 

vacuole,  48,  111,  134 
Olive,  59,  108,  202.  204,  208,  222 
Oliver,   17 

Oniscus  (crustacean),  255 
Onoclea  (fern),  288,  356,  372,  375 
Ontogenesis,  323,  324,  398,   L01,   L07,  111 
( >6apogamy,  315 

Oocytes  or  ovocytes,  220,  233,  274,  275 
Oogenesis,  221,  271.  275 
Oogonia  or  ovogonia,  221 
Oogonium  (of  plants),  179,  222.  223,  288 
Oomycetes,  289 
Open  spireme,  238 

Ophryotrochd  (annelid  worm  .  237,  211 
Orchis  (orchid).  104 
Organic  acids,  135 

Organism  as  a  whole.  12.  52,  1  M),  11 1 
Orman,  118 
Orthogenic i- .  in  1.   U0 
Orthoptera.  386,  393 
Oryza  (rice),  256 

OsciUatoria  (blue-green  alga  .  202,  204 
(  temosis  in  mitosis,  Is  I 
Osmunda  i  fern  .  apical  cell,  27 

reduction,  234,  236,  239,  240 

-c\-(lcterininal  ion,  373 

(  teterhout,  70,  248 
Otidea  (fungus),  Bl,  21 

(  >\ -arian  egg,  221,  27  1 

Overton,  E  .  13,  W,  ill.  220 
Overton,  J.  IV.  parthenogenesis,  314,  315 
prochromosomes,  159 


444 


INDEX 


Overton,  reduction,  223,  236,  248,  249 

somatic  pairing,  256,  257 
Ovum,  221,  274,  275,  276 

activation  of,  284  ff. 

organization  of,  328  ff. 
Oxidation,  and  chondriosomes,  123,  124 

in  egg,  285,  287 

and  nucleus,  70 

and  sex-determination,  368 
Oxychromatin,  64,  66,  70 


Pace,  300,  315 

Pachynema,  231,  232,  233 

Pachytene,  231,  233,  258 

Padina  (brown  alga),  223 

Palladin,  135 

Pallavicinia  (liverwort),  122,  220,  248 

Pangenesis  hypothesis,  398,  399 

Pahgenosomes,  258 

Pangens,  258,  399 

Parallelism       of       chromosomes       and 

characters,  342-343 
Paramitome,  33 
Paramecium,  30,  60,  207,  283 
Paramylum,  108,  112 
Parasynapsis,   228,   231,   233,   234,   235, 

243,  248,  250,  251,  257 
Parasyndese,  228 
Paratettix  (grasshopper),  391 
Parechinus  (sea  urchin),  326 
Parental  chromosome  sets,  254,  279,  340 
Paris  (angiosperm),  301 
Parmenter,  160,  163,  319 
Parthenogenesis,  314,  317,  364 

artificial,  284  ff. 
Passifiora  (angiosperm),  181 
Patterson,  213 
Paulmier,  95,  244,  245 
Payen,  11,  193 
Payne,  119,  359,  360 
Pea,  Mendelism,  336,  337 

linkage,  378,  384 
Pearl,  138 
Pearson,  52 
Pectates,  194 
Pectose,  194 
Pediastrum  (green  alga),   140,  331,  334, 

411 
Pedicellina  (flatworm),  255 
Pelargonium  (angiosperm),  104,  332 
Pellia  (liverwort),  82,  87,  88 
Pellicle,  45 


Pensa,  117,  122 
Pentose  and  pentosan,  41 
Peperomia  (angiosperm),  299 
Peptone,  40 

Peranema  (flagellate),  83 
Percnosome,  89 
Perforatorium,  273,  275,  277 
Perikaryoplasm,  177 
Perinium,  195 
Peripatus  (arthropod),  281 
Periplaneta  (cockroach),  239 
Periplasm,  289 
Periplast,  44 
Perispore,  195,  196 
Perivitelline  space,  277 
Perla  (stone-fly),  239 
Permeability,  37,  43,  287 
Peronospora  (fungus),  290 
Peziza  (fungus),  80,  81,  223,  291 
Pfeffer,  aleurone  grain,  134 

amitosis,  211 

cell  wall,  193 

ectoplast,  42 

oil  body,  110 

protoplasmic  connections,  47 
Pfitzner,  33,  155 
Pfliiger,  328 
Phaeophycese,  108,  222 
Phajus  (orchid),  106 
Phascum  (moss),  317 
Phases  in  colloids,  35 

of  mitosis,  145 
Phenotype,  339 
Phillips,  202 
Phosphatid,  117 
Phospholipin,  117 
Phosphoric  acid,  40 
Photosynthesis,  105,  107,  108 
Phragmatobia  (moth),  360,  361,  363 
Phragmidium  (fungus),  291,  312 
Phragmites  (angiosperm),  346 
Phragmoplast,  176 
Phrynotettix     (grasshopper),     156,     158, 

162,  233,  237,  243,  258,  392 
Phycocyanin,  104 
Phycoerythrin,  104 
Phycomyces  (fungus),  110,  186,  187,  355, 

373 
Phycomycetes,  289 
Phyllactinia  (fungus),  80,  81,  290,  291 
Phyllocladus  (conifer),  295 
Phylloglossum  (pteridophyte),  91 
PhyUopneuste  (bird),  274 


/\  l>i:\ 


H" 


Phylloxera  (aphid),  -'Us.  357,  364 
Phy8a  (snail),  276 
Physiological  units,  18,  398 
Physiology  of  fertilization.  284  ff. 

of  sex,  369  #. 
Physostegia  (angiosperm),  176,  225 
Phi/teh  phus  (angiosperm  >,  46 
Picard,  147.  230 
Pictet,  .",i 

Pieris  (butterfly),  61,  21 1 
Pig,  chromosomes,  165,  301 
Pigcera  (butterfly).  ,_>.7_* 
Pigeons,  sex-determination,  369,  376 
Pigments.  104.  105,  122.  13.").  136 
Pilobolus  (fungus),  187 
Pinguiada  (angiosperm),  158 
Pinney,  159 

Pinnotheres  (crustacean),  274 
Pin  us  (pine),  cell  plate,  176 

rhondriosomes,  121,  122 

fertilization,  294,  295 

protoplasmic  connections,  46 

reduction,  239 
Pisciola  (leech),  178 
Pisum  (pea),  61,  121,  122,  249,  256,  34* 

linkage  in,  378,  384 
Pits.  28,  191,  192 
Pin  a  aria  (flat  worm),  139,  237 
Plantago  (angiosperm),  372 
Plasma  membrane,  25,  42 
Plasmahaut.  42 

Plasmatic  microsomes,  205,  200 
Plasmodermal  blepharoplast,  95,  97 
Plasmodesmen,  46 
Plasmodium  (protozoan),  317 
Plasmogamy,  292 
Plasmosome,  25,  65 
Plastid,  26,  103  ff. 

individuality  of,  113 

inheritance,  331-334 

primordia,    115,    122,    124.    332, 
333 
Plastidome,  122 
Plastidule,  L8 
Plastochondria,  1 18,  121 
Plough,  395 

Plumbaqella  (angiosperm),  225 
Podocarpu8  (conifer),  213 

Podophyllum  (angiosperm  ».   1 .">'.»,  231 1 

Point  mutation,  351 

Polar  bodies,  15,  221,  275,  276 

in  parthenogenesis,  318,  319,  364 
Polar  nuclei,  299,  300,  301 


Polarity,  L38jf.   329,  334 

PoHanthet  (angiosperm  ,  110 

Polioplasm,  12 

Politis,  110 

Pollen  grain  uall.  r.ti; 

tube,  294,  298,  299 
Polygon*  lla  (angiospei  m  .  25 
Polypodium    fern  .  236 
Polysiphonia    (red   alga  .   centrosph 
79,  80 

fertilization.  289 

reduction,  223,  236 
PolysHchum  (fern  I,  316,  :;i7 
Polystigma  (fungus),  291 
Polytoma  (green  alga),  83,  84 
Polytomella  (green  alga  . 
Polytrichum  (mo-  .  88,  89,  221 
Popoff,  120,  259 
Porella  (liverwort),  87 
Postreduction,  245,  248 
Potato  starch.  107 
Pratt  and  Long.  237 
Preformation  theory.  :; 
Preissia  (liverwort),  82,  293 
Prenant,  184,  185 
Prereduction,  245,  2  1s 
Prevost  and  Dumas,  14,  15 
Primaniypus,  245 

Primula  (primrose),  239.  248,  346,  391 
Pringsheim,  14,  311 
Prionidus  (bug),  359,  360 
Pritchard,  372 
Prochromosomes,  158,  159 
Progamic  sex-determination.  357 
Promitosis,  Jus 

Promorphology  of  ovum,  328  jf. 
Pronucleus,  275,  276,  277 
Prophase,  somatic.  144,  11.").  150  153 

heterotypic,  231  ff. 
Protandry,  371 
Protein,  39,  40.  .".1.  134 

crystals,  135 
Protenor  (bug  .  360 
Prothallial  auclei,  294 
Protokaryon,  68 
Protoplasm,  chemical  uatui  r.  50 

colloidal  nature.  .",  I  ff.t  ."» 1 
doctrine,   1  I 

early  obeerval  ions,  6,  32  ff. 
physical  properties,  32 
senescence,  136 
structural  theori< 
substratum  of  life,  32,  Is  .//". 


446 


INDEX 


Protoplasm,  a  system,  32,  49,  52 

varieties  of,  41 
Protoplasmic  connections,  46 
Protoplasmic! ,  41 
Protoplast,  12,  24 
Protoxylem,  192 
Protozoa,  ectoplast,  44 

mitosis,  207  ff. 

nucleus  68 

reduction  222 
Pseudapogamy,  312 
Pseudapospory,  315 
Pseudoamitosis,  211 
Pseudogamy,  292 
Pseudomitosis,  204 
Pseudopodium,  45 
Psilotum  (pteridophyte),  239 
Pteridophytes,  fertilization,  292,  293 

sex-determination,  356 

spermatogenesis,  89  ff. 
Pteris  (fern),  248,  312,  317 
Puccinia  (rust  fungus),  312 
Punnett,  395 
Purkinje,  11 
Pustidaria  (fungus),  116 
Pyrenoid,  104,  108 
Pyronema  (fungus),  81,  224,  290,  291 
Pyrrochoris  (bug),  244 
Pythium  (fungus),  290 


Q 


Quadrille  of  centers,  280 
Quadripolar  division,  325 
Quadrivalent  chromosomes,  223 
Qualitative  division  of  chromosome,  227, 

229,  297,  401 
Quantitative  theory  of.  sex,  370  ff. 


R 


Rabbit,  212,  237 

Rabl,  157,  181,  279 

Raciborski,  110 

Ramlow,  291 

Random     assortment     of     chromosome 

pairs,  255 
Ranunculacese,  315 
Raphides,  134,  135 
Rat,  391 

vom  Rath,  211,  227,  244,  245 
Rauber,  326,  402 
Rawitz,  77 


Reboulia  (liverwort),  293 
Recessive,  338,  339,  351 
Recombination  of  factors,  343,  344 
Reducing  action  of  cytoplasm,  123 
Reduction   of   chromosomes,  in  animals, 
220  ff. 

with  chromosome  tetrads,  243  ff. 
.  denned,  229 

discovery,  219;  220 

modes  of,  230  ff. 

numerical,  229 

in  plant  groups,  222-225 

in  somatic  cells?     259 

Weismann's  theory  of,  226 

without  qualitative  change,  249 
Reduplication  hypothesis,  395 
Reed,  65,  70 

Refractive  body,  118,  274 
Regaud,  115,  117 
Regeneration,  69,  137,  406 
Regional  mutation,  351 
Reichert,  42 
Reinke,  64 

and  Rodewald,  39,  41 
Rejuvenescence,  137 
Remak,  10,  43 
Reserve  starch,  106,  107 
Resin,  194 

Respiration,  123,  124 
Resting  stage  of  nucleus,  144,  150,  151, 

157 
Reticular  theory  of  protoplasm,   33,  49 
Reticulum  of  nucleus,  25,  64,  144,  150, 

157 
Rhabdites  (nematode  worm),  189 
Rheum  (angiosperm),  134 
Rhizina  (fungus),  291 
Rhizonema,  83,  84 
Rhizoplast,  83 
Rhizopus   (mold  fungus),   116,   186,  187, 

290 
Rhumbler,  183 
Ribes  (gooseberry),  236 
Riccardia  (liverwort),  82,  236 
Riccia  (liverwort),  87,  293 
Richardia  (angiosperm),  159,  248,  249 
Richards,  158,  159,  213 
Ricinus  (castor  bean),  134 
Riddle,  369,  375 
Ritter,  411 
Rivett,  111 
Robertson,  A.,  295 
Robertson,  T.  B.,  189 


INDEX 


h; 


Robertson,  \\ .  R.  B.,  160,  L65,  237,  245, 
249,  257,  351,  393,  394 

Rosa  (rose  .  3  Is 

Rosaces,  315,  3  Is 

Elosen,  65 

Rosenberg,  L60,  1 « >  1 
apogamy,  3  i~>.  316 
chromosomes  in  Crepis,  3 19 
reduction,  236,  251,  252 

Rosenvinge,  16 

Rotifers,  318,  364,  368,  370 

Roux,  154,  156,  328,  384,  392 

Roux-Weismann  theory,  225,  391 

Rubber,  135 

Ruckert,  159,  227.  244.  245,  27'.' 

Rudolph,  123 

Rudolphi,  5 

Ruhland,  43 

Kussow,  46 

Rusts,  60,  224,  292 

Ruzicka,  07 

Rytz,  187 


S 


Sabaschnikoff,  2  Js 
Sachs,  12,  49,  62 
Safir,  382 
Sagitta,  256 
Saguchi,  96,  212 

Sakamura,    chromosomes    and    specie-. 
348 

deranged  mitosis,  212,  214,  259 

extruded  chromatin,  136 

false  tetrads,  248,  249,  259 

reduction,  236,  254 

somatic  mitosis,  160,   162,   164,   L65 
Salamandra     (salamander),    achromatic 
figure.  178,  IM 

attraction  sphere,  77 

chromosomes,  150 

reduction,  237 
Salix  iwillow  i.  372 
Salter,  107 
Salts,  135 

Sands,  81 

Sap  in  cell,   135 

Sapehin,  88,  114.  123 
Saprolegnia    fungus  .  187,  2 
Sarcode,  1 1 
Sargant,  211 
Sax.  298  -300 
Schacht,  13 


Schacke,  365 
Schaeflfer,  12 
-  banner,  87,  239,  372 
Schaudinn,  67,  207,  282,  317 
Schaxel,  1 16 
Scheben,  27  1 
Schellenberg,  2  17 
Scherrer,  L05,  113,  lis.  I 
Schikorra,  291 

Schiller.  213 

Schilling,  1 12 

Schimper,  12.  107,  108,  1 13 
Schleicher,  1  13 
Schleiden,  7.  8,  13 
Schleip,  237,  257 
Schmitz,  14,  ids 
Schneider,  A.,  33,  143 
Schneider,  II..  236,  257 

Schntt  lander.   S2,    86,   (Ml 

Schreiner,  231,  235,  237,  257,  258 

Schuberg,  96 

Schultze,  M..  11 

Schultze,  W.  H.,  70 

Schulz,  34'.) 

SchiirhorY,  66,  213 

Schwann,  8,  9,  1  5 

Schwarz,  6 1 
Schweigger-Seidel,  1  I 
Scolojn  ndra  (centipede  .  255 

Scolopemlrium  (fern  .  315,  -!17 
Scopolia  (angiosperm  .  61 
Scorpion,  119 
Scott,  202 

Sea    urchin,    artiticial    parthenogenesis 
284,  287 

blastOmeres,  328 

hybrids,  325  -127 
>rr>,u<\   contraction,  232,   233,   238,   2 
Secondary   thickening  of  cell   wall.   191 
Secretions,  28,  30,  I 
Secretory  cells,  30 

3  gmentation,      of      chromoson  in 

fertilization,  296 

of  egg,  188 

Of  spireme.    1  15,  239 

Segregation,  of  chromosomi 

Mendelian,      5  108,  U 

Bomal  ic,  352,  395 

Seifrii .  38,  39,  13 

Seiler,  360,  361,  363 

Selachians,  237 

Selaginella,  plastids,   103,  104,   105,   1 1  ; 
122 


448 


INDEX 


Selaginella,  spore  coat,  194,  195 
Senescence,  63,  136  ff. 
Sequoia  (conifer),  294,  295 
Sex,  chromosomes,  358^".,  380  ff. 

determination  of,  354  ff.,  373  ff. 

intergrades,  370  ff. 

linkage,  380  ff. 

Mendelian  interpretation  of,  366 

practical  control  of,  376 

quantitative  theory  of,  370 

ratio,  alteration  of,  367  ff. 

reversal,  370  ff. 
Sex-limited  characters,  381 
Sex-linked  characters,  381 
Sexual  reproduction  and  rejuvenescence, 

137 
Shaffer,  118,  120,  121 
Sharp,  64,  87,  90,  92,  147  ff.,  160,  176, 

225,  242,  254 
Shaw,  91,  316 

Shull,  A.  F.,  and  Ladoff,  368 
Shull,  G.  H.,  382 
Sida  (crustacean),  319 
Sieve  tube,  28 
Silica,  135,  194 
Silk  gland,  66 

Simocephalus  (crustacean),  371 
Simplex  chromosome  group,  340 
de  Sinety,  358 
Siphonese,  59 
Slime  globules,  203 
de  Smet,  150,  159,  160 
Smilacina  (angiosperm),  236,  239 
Smith,  B.  G.,  159,  160,  278,  279 
Smith,  G.  M.,  108 
Smith,  H.  L.,  79 
Sokolow,  119 
Sol,  35 
Somatic    mitosis,     143    ff. 

comparison  with  heterotypic,  228 

pairing  of  chromosomes,  256 

segregation,  352,  395 
Sordaria  (fungus),  81 
Sorodiscus  (slime  mold),  208,  209 
Spallanzani,  14 
Species,  origin  of,  347 
Specificity  of  chromosomes,  162,  250 
Spek,  189 

Spencer,  48,  49,  398 
Spermatia,  312 
Spermatid,  87,  220 
Spermatocyte,  220,  235 
Spermatogenesis,  in  animals,  95,  96,  220 


Spermatogenesis,  chondriosomes  in,  119, 
120 

in  man,  361,  362 

in  plants,  86  ff. 
Spermatogonia,  220 
Spermatozoids,  86  jf.,  288  ff.,  293,  294 
Spermatozoon,  discovery,  14 

in  fertilization,  277  ff. 

in  heredity,  324,  328 

structure,  273,  275 

types,  274 
Sphacelaria  (brown  alga),  79,  186 
Sphaerechinus  (sea  urchin),  325-327 
Sphcerocarpos  (liverwort),  355,  364,  365, 

372,  375 
Sphceroplea  (green  alga),  222 
Sphcerotheca  (fungus),  290 
Sphserraphides,  135 
Sphagnum  (moss),  248 
Spherome,  122 

Spinacia  (angiosperm),  248,  256 
Spinax  (fish),  237 
Spindle,  145,  147,  154,  175 
Spiral  stage  of  chromosome,  150,  152, 155 

wall  thickening,  28,  192 
Spireme,  145,  150,  153,  154,  232,  238 

in  fertilization,  300,  301 

in  heterotypic  prophase,  241-243 
Spirogyra  (green  alga),  amitosis,  211 

conjugation,  288 

cytokinesis,  186 

enucleated  cells,  69 

mitosis,  209 

nuclear  fusion,  14,  273 

plastid,  104 

pyrenoid,  104,  108 

reduction,  222 
Spirostomum  (protozoan),  61,  62 
Spitzer,  70 

Splitting    of  chromosomes,  in  first  em- 
bryonal division,  295,  300;  in  matu- 
ration, 236,  239,  242,  258;  in  somatic 
mitosis,  153,  242,  392 
Spoehr,  41 
Spore  membrane,  195,  196 

tetrads,  223-225,  355 

walls,  194 
Sporocyte,  225,  232 

Sporodinia  (mold  fungus),  110,  187,  290 
Sporogenesis,    in   various    plant    groups, 
222  ff. 

behavior  of  plastids  in,  114 

without  reduction,  315,  316 


INDEX 


149 


Sporogenesis,     and     Bex-determination, 

:;.").")  356 
Sporophytio  budding,  312,  315 
Sprengel,  ( '.  K.,  L3 
Sprengel,  K.,  5 
Stains,  117 

Stangeria  (cycad  I,  294,  298 
Staphylea  (angiosperm),  239 
St  a  nli.  106,  107,  112,  134 
Starfish,  286,  287 
Steil,  28S,  313,  314 
Stein,  61 

Stemonitis  (slime  mold),  85 
Stentor  (protozoan),  60,  62,  69 
Stevens,  F.  L.,  289,  290 
Stevens,  N.  M.,  256,  358,  360,  361 
Stomps,  155,  248,  256,  345-347 
Storage,  66,  106,  133 
Stout,  159,  372 
Strasburger,  amitosis,  213 

apogamy   and   apospory,   311,   315, 
316 

blepharoplast,  85 

cell-division,  143 

cell  wall,  190,  193 

centrosphere,  76,  79,  80 

chromomeres,  155,  156,  258 

chromosome  number,  163,   164 

chromosomes  (somatic),  149,   158 

cytokinesis,  186,  190 

ectoplast  43 

eyespot,  111,  112 

fertilization,  300 

mitosis  in  myxomycetes,  208 

nuclear  fusion,  14,  273 

nuclei  by  division  only,  10 

nucleolus,  66 

nucleoplasmic  ratio,  62 

nucleus  in  heredity,  16,  324,  327 

of  Marsih'd.  210 

protoplasm,  34 

protoplasmic  connections,  47 

reduction,  220.  222.  236 

Bex-determination,  355,  356,  364 

somatic  pairing,  256 

spindle,  L80,  181 

spore  wall,  19  1 

starch,  106 

tct raploidy,  345 
Siratiotes  (angiosperm),  212 
Streaming  in  cell-division,  ls;!.  189 
Strepsinema,  231,  233,  238 
Strepsitene,  231,  233 

29 


Stroma,  105 

starch,  109 
Strong,  376 
Strongyloa  ntroi  ■         i  urchin  .  287, 

327 
Structure  and  function,  37  I 
Studnicka,  96 
Sturgeon,  274 
Sturtevanl .  371 .  376,    389 
Styela  (ascidian  .  329 
Stypocavlon  (brown  al  ii-.i  .  79 
Suberin,  194 
Sugars,  106,  135.  136 
Supernumerary  chromosomt       >fi  jj. 
Surface  tension  in  cytokinesis,   lss 
Surirella  (diatom),  7'.) 
Suspensoid,  35 
Sutton,  158,  160.  228,  245,  251,  252,  - 

255 
Swarczewsky,  2s  1 
Swarm  spores  and  colony,  1  lo.  331,  334, 

111 
Swingle.  I).  B.,  L86,  187 
Swingle.  W.  T.,  79,   186 
Sykes,  256 

Symmetry,  139,  329,  334 

Synapsis.  227.  233,  251  ffi. 

and  degeneration,  259 
nature  of  union,  256,  384,  394 

Synaptene,  231,  233 

Synaptic  mates,  relationship,  251   251 

Synchytrium  (fungus),  62,  ls7 

Syndiploid  nuclei.  259,  260 

Synergids.  299 

Syngamic  sex-determination,  357 

Syngamy,  222 

Synizesis,  231,  233,  237,  238,  255 
in  somat  ic  cells?     259 

Synkaryon,  27s.  282,  292 

Synochocystis  (blue-green  alga  .  205 


T 


T&ckholm,  248,  348 

Tahara,  256 

Tamus  (angiosperm  I,  17 

Tangl,  16,  17 

Tannin,  P'l 

Tapetal  cells,  59,  210 

Plasmodium,  194,  196 
Taraxacum  (dandelion  .  3 15 
Tassemenl  polaire,  149 
Taxodium  (conifer  .  295 


450 


INDEX 


Taxus  (conifer),  122,  295 

Taylor,  M.,  256 

Taylor,  W.  R.,  176 

Teleosts,  160,  237 

Teliospore  or  teleutospore,  292 

Telophase,  somatic,  144,  145,  148 

Telophasic  splitting?     149,  240,  242 

Telosynapsis,  228,  239,  244,  245,  248 

Tennent,  160 

Teosinte,  349 

Terletzki,  46 

Terni,  118 

Tertiary  thickening  of  wall,  191 

Tetrads,  of  cells,  220/. 

of    chromosomes,     230,     234,     239, 
243  Jf.;  in  plants,  248,  249 
Tetraploidy,  317,  345 
Tetraspora  (green  alga),  109 
Tetraspores,  223 
Tettigidse,  243 
Thalidrum  (angiosperm),  159,  236,  248, 

315 
Thelygonium  (angiosperm),  236 
Theophrastus.  1 
Thorn,  91 

Thompson,  52,  333 
Thomson,  369,  404 
Thuja  (conifer,  arbor  vitse),  239,  294 
Thuret,  14 

Thysanozoon  (flat worm),  237 
Timberlake,  85,  105,  109,  187,  190 
Tischler,  chondriosomes,  118 

gigantism,  346 

prochromosomes,  159 

reduction,  236 

spore  coat,  196 
Tissue  cultures,  138,  146 
Toad  eggs,  368,  370 
Tolypothrix  (blue-green  alga),  203 
Tomopteris  (annelid  worm),  235 
Tonoplast,  25,  47,  111 
Torreya  (conifer),  295 
Torus,  191,  192 
Townsend,  69 

Toxopneustes  (sea  urchin),  160 
Tracheid,  28,  191,  192,  345 
Trachelomonas  (protozoan),  112 
Tradescantia    (angiosperm),    protoplasm, 
6,  25,  33,  37 

reduction,  236,  239 

somatic  mitosis,  147,  148,  150,  152, 
242 
Tragopogon  (angiosperm),  239 


Transverse  division  of  chromosome,  227 

Tretjakoff,  248 

Treub,  190 

Treviranus,  5,  6 

Trichites,  107 

Trichocyst,  30,  45 

Trichogyne,  289 

Tricyrtis  (angiosperm),  248 

Trillium    (angiosperm),    149,    155,    300, 

301 
T rimer otropis  (grasshopper),  255 
Triple  fusion  in  angiosperms,  299,  301 
Triploidy,  347 

Triticum     (wheat),     chromosomes    and 
species,  348 

fertilization,  299 

synapsis  in  hybrids,  252,  253 
Triton  (salamander),  274 
Trollius  (angiosperm),  236 
Trondle,  109,  222 
Trophochromatin,  208,  209 
Trophoplasm,  34,  181,  400 
Trow,  290,  395,  396 
Trypanosoma  (flagellate),  45,  84 
Tschernoyarow,  160,  161,  253 
Tschirch/ 107,  134 
Tsuga  (conifer,  spruce),  295 
Tulasne,  13 

Tulipa  (angiosperm),  298 
Tumors,  163 
Tunicata,  237 
Tupper  and  Bartlett,  345 
Turbellarian  egg,  329 
Twins,  357 
Twiss,  118 
Tyloses,  193 
Tyndall  effect,  35 


U 


Ulothrix  (green  alga),  104,  222,  288,  331 

Undulating  membrane,  45,  274 

Unger,  9 

Unio  (mollusk),  280 

Unna,  34 

Uredineje,  60,  224,  292 

Uromyces  (rust  fungus),  312 


Vacuole,  25,  47,  60,  111,  135 

contractile,  30,  44,  46,  84 
Vacuome,  122 


INDEX 


451 


Vandendries,  224 

Vanessa  (butterfly),  61 
Van  Hook,  82 
Vanilla  (angiosperm  I,  !<)*.» 
Varieties,  origin  of,  3-17 
Vaucheria  (green  alga),  60,  85 
Vegetative  apogamy,  314,  315 

nnil  at  ion,  351 
Vejdowsky,  67,  250,  257 
Velten,  33 

Vermiform  nuclei,  298,  299 
Verworn,  42,  50,  326 
Vesper  ago  (bat),  274 
V tela  (horse  bean),  chromosome  comple- 
ment, 160,  161 

deranged  mitosis,  211,  212 

reduction,  235,  236,  238,  254 

somatic  mitosis,  147,  153,  242 
Vines,  311/. 
Virchow,  10,  15 

Viscosity  of  protoplasm,  39,  62,  189 
Viscum  (mistletoe),  47 
Vital  units,  48,  398  ff. 
Vitalism,  49,  52,  409,  412 
Vitelline  membrane,  286 
Vo'inov,  118,  119 
Volutin,  41 

Volvocaceae  (green  algae),  112 
von  Voss,  257 
de  Vries,  intracellular  pangenesis,  399 

(Enothera  mutants,  344,  347,  349 

pangens,  49,  258,  399 

tonoplast,  47 


W 


W-chromosome,  363 

Wager,  13,  108,  112,  290 
Wakker,  109,  110 
Waldeyer,  144,  326 
Walker,  224 

Wall  of  roll,  26,  28,  190  jf. 
Walton,  248,  279,  405 
Warburg,  285,  287 
Wasielewski,  211,  212,  213 
Wassilief,  118 
Watase,  181 

Water  in  protoplasm,  37 
Webber;  83,  92,  93,  288,  294,  302 
Weinstein,  389 
Weinzieher,  236 
Weismann,  16,  49,  225 
parthenogenesis,  31 8 


Weismann,  reduction,  226 

theories   of   heredity   ami   develop- 
ment, 391,  loo  ]T. 
Wells,  H» 

Welsford,  290,  298,  299,  312,  325 
Weniger,  298  :;<)1 
Wenrich,   156.    158,    162,   233,   237,   243, 

J.-.7.  lios.  392, 
West  and  Lechmere,  136 
Wheeler,  279 
Wheldale,  135 
Wherry,  70 
White,  384 
Whitman,  369 
Whitney,  357,  36s.  370 
Wieman,  361,  362 
Wiesner,  48,  109,  193 
Wildman,  lis.  120.  121.  281 
Wille,  46 

Williams,  C.  L.,  1M 
Williams,  J.  L.,  80.  s2.  223 
Wilson,  E.  B.,  abnormal  eleavage,  328 

achromatic  figure,  181-1  v"» 

chemical  nature  of  gene,   112.   114 

chondriosomes,  119 

chromatin  and  cytoplasm,  69 

chromosomes  and  heredity,  396  397 

conjugation  in  Paramcecium,  284 

crossing  over,  393,  394 

genetic  continuity,  323 

hyaloplasm  sphere,  76 

individuality  of  chromosome,  157 

protoplasm,  34,  49 

sex-chromosome-.  :;."i^  ff. 

somatic  chromosomes,  152 

spermatozoon,  275 
Wilson,  H.  V.,  1 38 
Wilson,  M.,  88,  89 
Wilson  and  Morgan,  386,  393 
Winge,  208,  209 
von  Winiwarter,  centrosomes,  77 

chondriosomes,  123 

chromosomes  in  man.  362 

reduction,  228,  231,  237 
von  Winiwarter  and  Sainmont,  257 
Winkler,  62,  Ml  I 
win  Wisselingh,  210 
Wodsedalek,  361 
Wdhler,  50 
Wolfe,  80,  223 
WolflF,  i.  5 

Wollenweber,  ill.  112 
Woodburn,  2 


452 


INDEX 


Woolery,  239 

Wort  man,  56 
Woyciki,  295 

Wuist,  372 


X 


X-chromosome,  359  jf.,  380  ff. 
Xanthophyll,  104,  105 
Xenia,  302 
Xylaria  (fungus),  291 
Xyris  (grass),  236 


Y-chromosome,  359  ff.,  380  #.,  390 

Y.i  manouchi,  achromatic  figure,  176,  179 

apogamy,  314 

blepharoplast,  91 

centrosomes,  79,  80,  82 

eyespot,  111 

fertilization,  289,  293 

pyrenoid,  109 

reduction,  222,  223,  232,  236 
Yampolsky,  372 
Yeasts,  206,  210,  292 
Yolk,  134 
Young,  213 
Yucca  (angiosperm),  256 


Z-chromosome,  363 
Zacharias,  65,  193,  202,  404 


Zade,  349 

Zamia  (cycad),  93,  288,  294 
Zanardinia  (brown  alga),  111,  222 
Zea  (maize),  65,  122 

aleurone  inheritance,  333 

chlorophyll  inheritance,  332 

chromosomes,  349 

crossing  over,  391 

linkage  groups,  383 
Zentralkorner,  203 
Zettnow,  67 
Zigzag  stage  of  chromosomes,  151,  152, 

153 
Zimmermann,  110,  193 
Zoja,  164 

Zoogonus  (worm),  237,  244,  245 
Zoospores,  85,  86,  112,  137,  222 
Zukal,  202 
Zweiger,  239 
Zygnema  (green  alga),  fertilization,  113 

mitosis,  210 

plastid,  103,  106,  108,  109 

pyrenoid,  109 

reduction,  222 
Zygogenetic  eggs,  318,  319 
Zygomycetes,  290 
Zygonema,  231,  233 
Zygospore,  113,  222 
Zygote,  222,  223 
Zygotene,  231,  233 
Zymogen,  66 


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